Science, Tech, Math Science A to Z Chemistry Dictionary Look Up Definitions of Important Chemistry Terms Share Flipboard Email Print Chemistry is full of precise definitions!. COLIN CUTHBERT/SCIENCE PHOTO LIBRARY / Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry in Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate by Anne Marie Helmenstine, Ph.D. Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. Updated May 08, 2018 This alphabetical chemistry dictionary offers definitions and examples of important chemistry and chemical engineering terms. For each term, a brief definition is given. Each link leads to a more comprehensive discussion of the word. Additional definitions are also available. A- Absolute Alcohol to Azimuthal Quantum Number Alkalinity is a measure of how basic a substance is. JazzIRT / Getty Images absolute alcohol - common name for high purity ethanol or ethyl alcohol. absolute error - expression of the uncertainty or inaccuracy of a measurement. absolute temperature - temperature measured using the Kelvin scale. absolute uncertainty - the uncertainty of a scientific measurement, given in the same units as the measurement. absolute zero - the lowest possible state at which matter can exist, 0 K or -273.15°C. absorbance - measure of the amount of light absorbed by a sample. absorption - process by which atoms, ions, or molecules enter a bulk phase. absorption spectroscopy - technique used to determine concentration and structure of a sample based on which wavelengths of liquid are absorbed. absorption spectrum - graph of amount of absorption as a function of wavelength. absorptivity - absorption cross section of extinction coefficient, which is the absorbance of a solution per unit path length and concentration. accuracy - the closeness of a measurement to a true or accepted value. acid - a chemical species that accepts electrons or donate protons or hydrogen ions. acid anhydride - a nonmetal oxide that reacts with water to form an acidic solution. acid-base indicator - a weak acid or weak base that changes color when the concentration of hydrogen or hydroxide ions changes in an aqueous solution. acid-base titration - a procedure to find the concentration of an acid or base by reacting a known concentration with the unknown until the equivalence point is reached. acid dissociation constant - Ka - a quantitative measure of how strong an acid is. acidic solution - an aqueous solution with a pH less than 7.0. actinides - Usually, the actinides are considered to be elements 90 (thorium) through 103 (lawrencium). Otherwise, the actinides are defined according to their common properties. actinium - the name for the element with atomic number 89 and is represented by the symbol Ac. It is a member of the actinide group. activated complex - an intermediate state at the maximum energy point on the reaction path that occurs as reactants are being converted into product in a chemical reaction. activation energy - Ea - the minimum amount of energy needed for a chemical reaction to occur. active transport - the movement of molecules or ions from a region of lower concentration to higher concentration; requires energy activity series - list of metals ranked in order of decreasing activity, used to predict which metals displace others in aqueous solutions. actual yield - the quantity of product experimentally obtained from a chemical reaction. acute health effect - the effect caused by initial exposure to a chemical. acyl group - a functional group with the formula RCO- where R is bound to carbon via a single bond. adsorption - the adhesion of a chemical species onto a surface adulterant - a chemical that acts as a contaminant in the context of another substance's purity. aether - a medium believed to carry light waves in the 18th and 19th century. air - the mixture of gases that make up the Earth's atmosphere, consisting mainly of nitrogen, with oxygen, water vapor, argon, and carbon dioxide. alchemy -Several definitions of alchemy exist. Originally, alchemy was an ancient tradition of sacred chemistry used to discern the spiritual and temporal nature of reality, its structure, laws, and functions. alcohol - a substance that contains an -OH group attached to a hydrocarbon. aliphatic amino acid - amino acid that has an aliphatic side chain. aliphatic compound - an organic compound containing carbon and hydrogen joined into straight chains, branches chains, or non-aromatic rings. aliphatic hydrocarbon - a hydrocarbon containing carbon and hydrogen joined into straight chains, branches chains, or non-aromatic rings. alkali metal - any element found in group IA (first column) of the periodic table. alkaline - an aqueous solution with a pH greater than 7. alkalinity - a quantitative measure of a solution's ability to neutralize an acid. alkene - a hydrocarbon containing a double carbon-carbon bond. alkenyl group - the hydrocarbon group formed when a hydrogen atom is removed from an alkene group. alkoxide - an organic functional group formed when a hydrogen atom is removed from the hydroxyl group of an alcohol when it is reacted with a metal. alkoxy group - functional group containing an alkyl group bonded to oxygen. allotrope - a form of an elemental substance. alloy - substance made by melting together two or more elements, at least one of which must be a metal. alpha decay - spontaneous radioactive decay which produces an alpha particle or helium nucleus. alpha radiation - the ionizing radiation released from radioactive decay emitting an alpha particle. aluminum or aluminium - the name for the element with atomic number 13 and is represented by the symbol Al. It is a member of the metal group. amalgam - any alloy of mercury and one or more other metals. americium - radioactive metal with element symbol Am and atomic number 95. amide - functional group containing a carbonyl group linked to a nitrogen atom. amine - compound in which one or more hydrogen atom in ammonia is replaced by an organic functional group. amino acid - an organic acid containing a carboxyl (-COOH) and amine (-NH2) functional group along with a side chain. amorphous - term describing a solid that does not have crystalline structure. amphiprotic - species that can both accept and donate a proton or hydrogen ion. amphoteric - substance capable of acting as either an acid or a base. amphoteric oxide - oxide that can act as either an acid or a base in a reaction to produce a salt and water. amu - atomic mass unit or 1/12th the mass of an unbound atom of carbon-12. analytical chemistry - chemistry discipline that studies the chemical composition of materials and tools used to examine them. angstrom - unit of length equal to 10-10 meters. angular momentum quantum number - ℓ, the quantum number associated with the angular momentum of an electron. anhydrous - describes a substance that does not contain water or else is as concentrated as it can get. anion - an ion with a negative electrical charge. anode - electron where oxidation occurs; positive charged anode antibonding orbital - molecular orbital with an electron outside the region between the two nuclei. anti-Markovnikov addition - an addition reaction between an electrophilic compound HX and either an alkene or alkyne in which the hydrogen atom bonds to the carbon with the least number of hydrogen atoms and X bonds to the other carbon. antimony - Antimony is the name for the element with atomic number 36 and is represented by the symbol Kr. It is a member of the metalloid group. anti-periplanar - periplanar conformation where the dihedral atom between atoms is between 150° and 180°. aqueous - describes a system containing water. aqueous solution - a solution in which water is the solvent. aqua regia - mixture of hydrochloric and nitric acids, capable of dissolving gold, platinum, and palladium. argon - Argon is the name for the element with atomic number 18 and is represented by the symbol Ar. It is a member of the noble gases group. aromatic compound - an organic molecule that contains a benzene ring. Arrhenius acid - species that dissociates in water to form protons or hydrogen ions. Arrhenius base - species that increases the number of hydroxide ions when added to water. arsenic - metalloid with element symbol As and atomic number 33. aryl - a functional group derived from a simple aromatic ring when one hydrogen is removed from the ring. astatine - Astatine is the name for the element with atomic number 85 and is represented by the symbol At. It is a member of the halogen group. atom - the defining unit of an element, which cannot be subdivided using chemical means. atomic mass - average mass of atoms of an element. atomic mass unit (amu) - 1/12th the mass of an unbound atom of carbon-12, used to represent atomic and molecular masses. atomic number - the number of protons in the nucleus of an atom of an element. atomic radius - value used to describe the size of an atom, usually half the distance between two atoms just touching each other. atomic solid - solid in which atoms are bonded to other atoms of the same type. atomic volume - volume occupied by one mole of an element at room temperature. atomic weight - average mass of atoms of an element. atmosphere - surrounding gases, such as the gases surrounding a planet that are held in place by gravity. ATP - ATP is the acronym for the molecule adenosine triphosphate. Aufbau principle - idea that electrons are added to orbitals as protons are added to an atom. austenite - the face-centered cubic crystalline form of iron. Avogadro's Law - relation that states equal volumes of all gases contain the same number of molecules at the same pressure and temperature. Avogadro's number - the number of particles in one mole of a substance; 6.0221 x 1023 azeotrope - a solution that retains its chemical composition when distilled. azimuthal quantum number - the quantum number associated with the angular momentum of an electron, determining the shape of its orbital. B Definitions - Background Radiation to Buffer Boiling occurs when the vapor pressure of a liquid exceeds atmospheric pressure. David Murray and Jules Selmes / Getty Images background radiation - radiation from external sources, typically from cosmic radiation and radioisotope decay. back titration - titration in which the analyte concentration is determined by reacting it with a known quantity of excess reagent. balanced equation - chemical equation in which the number and type of atoms and the electric charge is the same on both the reactant and product sides of the equation. Balmer series - the part of the hydrogen emission spectrum for electron transitions n=2 and n>2, There are four lines in the visible spectrum. barium - alkaline earth metal with element symbol Ba and atomic number 56. barometer - instrument used to measure atmospheric pressure. base - chemical species that either accepts protons or else donates electrons or hydroxide ions. base anhydride (basic anhydride) - a metal oxide formed from the reaction between water and a basic solution. base metal - any metal besides a precious or noble metal used for jewelry or in industry. basic - alkaline or having a pH > 7. basic solution - aqueous solution containing more hydroxide ions than hydrogen ions; solution with pH > 7. Beer's law (Beer-Lambert Law) - law that states the concentration of a solution is directly proportional to its light absorbance. berkelium - radioactive metal with element symbol Bk and atomic number 97. beryllium - alkaline earth metal with element symbol Be and atomic number 4. beta decay - type of radioactive decay that results in spontaneous emission of a beta particle. beta particle - an electron or positron emitted during beta decay. beta radiation - ionizing radiation from beta decay in the form of an energetic electron or positron. binary acid - an acidic binary compound in which one element is hydrogen and the other element is another nonmetal. binary compound - a compound made up of two elements (e.g., HF). binding energy - energy needed to remove an electron from an atom or to separate a proton or neutron from the atomic nucleus. biochemistry - Biochemistry is the chemistry of living things. bismuth - Bismuth is the name for the element with atomic number 83 and is represented by the symbol Bi. It is a member of the metal group. bitumen - natural mixture of polycyclic aromatic hydrocarbons (PAHs). black light - a lamp that emits ultraviolet radiation or the invisible radiation emitted by it. block copolymer - copolymer formed by repeating monomer subunits. bohrium - transition metal with element symbol Bh and atomic number 107. boiling - phase transition from the liquid to gas state. boiling point - temperature at which a liquid's vapor pressure is equal to the external gas pressure. boiling point elevation - the increase in a liquid boiling point caused by adding another compound to it. bond - a chemical link formed between atoms in molecules and molecules and ions in crystals. bond angle - the angle formed between two adjacent chemical bonds within the same atom. bond-dissociation energy - energy required to homolytically break a chemical bond. bond energy - quantity of energy needed to break one mole of molecules into component atoms. bond enthalpy - enthalpy change resulting when one mole of bonds in a species are broken at 298 K. bond length - the equilibrium distance between atomic nuclei or groups of nuclei that share a chemical bond. bond order - a measure of the number of electrons involved in chemical bonds between two atoms in a molecule; usually equal to the number of bonds between the atoms. boron - Boron is the name for the element with atomic number 5 and is represented by the symbol B. It is a member of the semimetal group. Boyle's law - ideal gas law that states the volume of a gas is inversely proportional to its absolute pressure, assuming constant temperature. branched chain alkane - an alkane with alkyl groups bonded to the central carbon chain. The molecules are branched, but all C-C bonds are single bonds. brass - Brass is defined as an alloy of copper and zinc. bromine - Bromine is the name for the element with atomic number 35 and is represented by the symbol Br. It is a member of the halogen group. Bronsted-Lowry acid - species that yields hydrogen ions. Bronsted-Lowry base - species that accepts hydrogen ions in a reaction. bronze - Bronze is an alloy of copper, usually containing tin as its main addition. buffer - either a weak acid and its salt or else a weak base and its salt that form an aqueous solution that resists pH changes. C - Cadmium to Current The Celsius scale is a common temperature scale in chemistry. Indeed / Getty Images cadmium - Cadmium is the name for the element with atomic number 48 and is represented by the symbol Cd. It is a member of the transition metals group. caffeine - Caffeine is a chemical substance naturally found in tea and coffee and added to colas. calcium - Calcium is the name for the element with atomic number 20 and is represented by the symbol Ca. It is a member of the alkaline earth metal group. calorie - unit of thermal energy; the amount of energy required to raise the temperature of 1 gram of water 1 degree C or K at standard pressure. calorimeter - instrument designed to measure heat flow of a chemical reaction or physical change. capillary action - the spontaneous floe of liquid into a narrow tube or porous material. carbon - Carbon is the name for the element with atomic number 6 and is represented by the symbol C. It is a member of the nonmetal group. carbonate - an ion consisting of one carbon bonded to three oxygen atoms (CO32-) or a compound containing this ion. carbonyl - functional group consisting of a carbon atom double bonded to oxygen, C=O. carboxyl group - functional group consisting of a carbon double bonded to oxygen and single bonded to a hydroxyl (-COOH). catalyst - substance that increases the chemical reaction rate by decreasing its activation energy. catenation - binding of an element to itself via covalent bonds, forming a chain or ring cathode - electrode where reduction occurs; usually the negative electrode. cathode ray tube - a vacuum tube with a source of electrons, a fluorescent screen, and means of accelerating and deflecting the electron beam. cation - ion with a positive electrical charge. Celsius temperature scale - temperature scale where 0°C and 100°C are defined as the freezing and boiling points of water, respectively. cerium - rare earth metal with element symbol Ce and atomic number 58. cesium - Cesium is the name for the element with atomic number 55 and is represented by the symbol Cs. It is a member of the alkali metal group. cetane number (CN) - value that describes the combustion quality of diesel fuel, based on the delay between injection and ignition. chain reaction - set of chemical reactions in which products become reactants of another reaction. charge - an electrical charge, a conserved property of subatomic particles determining their electromagnetic interaction. Charles's law - ideal gas law that states the volume of an ideal gas is directly proportional to absolute temperature, assuming constant pressure. chelate - organic compound formed by bonding a polydentate ligand to a central metal atom, or the act of forming such a compound. chemical - any matter or substance that has mass. chemical change - process by which one or more substances are altered to form new substances. chemical energy - energy contained in the internal structure of an atom or molecule. chemical equation - description of a chemical reaction, including the reactants, products, and direction of the reaction. chemical equilibrium - state of a chemical reaction where the concentration of the reactants and products remains stable over time. chemical formula - expression which states the number and type of atoms in a molecule. chemical kinetics - the study of chemical processes and rates of reactions. chemical property - characteristic which may be observed when matter undergoes a chemical change. chemical reaction - a chemical change in which reactants form one or more new products. chemical symbol - one- or two-letter representation of a chemical element (e.g., H, Al). chemiluminescence - light emitted as a result of a chemical reaction chemistry - study of matter and energy and the interactions between them Cherenkov radiation - Cherenkov radiation is the electromagnetic radiation emitted when a charged particle moves through a dielectric medium faster than the velocity of light in the medium. chiral center - the atom in a molecule bonded to four chemical species, allowing optical isomerism. chirality - Chirality or chiral describes a nonsuperimposable mirror image, like left and right hands. Usually in chemistry the term is used to describe a pair of molecules that have the same formulas, but form a pair of structures. chlorine - halogen with atomic number 17 and element symbol Cl. chlorofluorocarbon - A chlorofluorocarbon or CFC is a compound that contains atoms of chlorine, fluorine, and carbon. chromatography - group of techniques used to separate mixture components by passing the mixture through a stationary phase. chromium - Chromium is the name for the element with atomic number 24 and is represented by the symbol Cr. It is a member of the transition metals group. closed system - thermodynamic system in which mass is conserved within the system, but energy can freely enter or exit. coagulation - the gelling or clumping of particles, usually in a colloid. cobalt - transition metal that is atomic number 27 with element symbol Co. coenzyme - substance that works with an enzyme to aid its function or initiate its action. cohesion - measure of how well molecules stick to each other or group together. collagen - an important family of proteins found in humans and other animals, found in skin, cartilage, blood vessels, and tendons. colligative properties - properties of a solution that depend on the number of particles in a volume of solvent. colloid - a homogenous mixture in which dispersed particles do not settle out. combined gas law - law which states the ratio of the product of pressure and volume, divided by the absolute temperature, is a constant value. combination reaction - reaction in which two reactants combine to form a single product. combustion - chemical reaction between a fuel and oxidizer that yields energy (usually heat and light). common-ion effect - suppressing effect an electrolyte has on the ionization of another electrolyte that shares a common ion. compound - chemical species formed when two or more atoms form a chemical bond. complex ion - ion in which a central metal ion is bonded to one or more ions or molecules. concentrated - having a large ratio of solute to solvent. concentration - an expression of the quantity of a substance in a defined volume. condensation - state of matter change from vapor phase to liquid phase. condensation reaction - chemical reaction in which one of the products is water or ammonia, also known as a dehydration reaction. condensed formula - chemical formula in which atom symbols are listed in the order they appear in the molecular structure, with limited bond dashes. conductor - material which permits the flow of energy (e.g., electrical conductor, thermal conductor). conformer - an isomer that differs from another isomer by rotation around a single bond. congener - member of the same group of elements of the periodic table (e.g., iodine and chlorine). conjugate - multiple chemistry definitions, referring to Bronsted acids and bases, a compound formed by combining other compounds, or the overlap of p-orbitals across a sigma bond. conjugate acid - HX, a compound differing from a base X by a proton. conjugate base - the species that gains a proton in an acid-base reaction. conservation of energy - law which states energy can change forms but may not be created or destroyed. conservation of mass - law that states, in a closed system, matter can change forms but not be created or destroyed. controlled variable - variable that a scientist holds constant in an experiment; the control or constant variable conversion factor - numerical ratio that converts a measurement from one unit into another. coordinate bond - covalent bond between two atoms in which one atom supplies both electrons for the bond. coordination compound - compound containing one or more coordinate bonds. coordination number - number of atoms bonded to a central atom. copernicium - radioactive element with symbol Cn and atomic number 112. copper - Copper is the name for the element with atomic number 29 and is represented by the symbol Cu. It is a member of the transition metals group. corrosion - irreversible damage to a material or tissue due to a chemical reaction. corrosive - having the power to cause irreversible chemical damage upon contact. Coulomb's law - law which states the force between two charges is proportional to the quantity of both charges and inversely proportional to the square of the distance between them. covalent bond - chemical link between atoms or ions in which the electron pairs are more or less evenly shared between them. covalent compound - molecule that contains covalent chemical bonds. covalent radius - half the diameter of the part of an atom that participates in a covalent bond. crenation - forming a scalloped shape upon exposure to a hypertonic solution. critical point - critical state; point at which two phases of matter become indistinguishable from one another. cyrogenics - study of matter at extremely low temperatures crystal - matter in which atoms, ions, or molecules are packed into an ordered, repeating three-dimensional pattern. crystal field splitting - the difference in energy between the d orbitals of ligands. crystallize - solidification of matter into the highly ordered form of a crystal. curium - radioactive metal with element symbol Cm and atomic number 96. current - rate of flow of electricity. D - Dalton's Law to Dysprosium Dry ice is the name for solid carbon dioxide. Jasmin Awad / EyeEm / Getty Images Dalton's Law - relation stating the total pressure of a gaseous mixture equals the sum of the partial pressure of the component gases. darmstadtium - Darmstadtium is the name for the element with atomic number 110 and is represented by the symbol Ds. Darmstadtium was formerly known as ununnilium with symbol Uun. It is a member of the transition metals group. dative bond - covalent bond between atoms in which one atom provides both electrons for the bond. daughter isotope - product formed after a radioisotope (the parent) undergoes radioactive decay. de Broglie Equation - equation describing the wave properties of matter, stated as wavelength equals Planck's constant divided by the product of mass and velocity. decantation - method of separating mixtures by removing the liquid layer from a precipitate. decomposition reaction - chemical reaction in which a single reactant yields two or more products. deflagration - type of combustion in which flame propagation is less than 100 m/s and overpressure is less than 0.5 bar. dehydration reaction - chemical reaction between two compounds in which one of the products is water. deliquescence - process by which a soluble substance picks up water vapor from the atmosphere to form a solution. delocalized electron - any electron in an ion, atom, or molecule that is no longer associated with a particular atom or single covalent bond. density - mass per unit volume. dependent variable - variable being measured (tested) in response to changing the independent variable. deposition - settling of sediment or particles onto a surface or the phase change from the vapor to solid phase. deprotonation - chemical reaction in which a radical removes a proton from a molecule. derived unit - an SI unit made from a combination of the base units (e.g., Newton is kg·m/s2). desiccant - chemical agent that picks up water, often used for drying. desublimation - phase change from vapor to solid. detergent - cleaning agent with general structure R-SO4-, Na+, where R is a long-chain alkyl group. diamagnetic - not attracted to a magnetic field, generally because the material does not contain unpaired electrons. diffusion - movement of a fluid from a region of higher concentration to lower concentration. dilute - solution containing a small amount of solute relative to the amount of solvent. dipole - a separation of electrical or magnetic charges. dipole moment - measure of the separation of two opposite electrical charges. diprotic acid - acid that can donate two hydrogen atoms or protons per molecule in an aqueous solution. direct proportion - relation between two variables such that their ratio is a constant value. disaccharide - carbohydrate formed when two monosaccharides bond, removing a molecule of water from their structure. displacement reaction - chemical reaction in which the cation or anion of one reactant is replaced by one from another reactant. disproportionation - chemical reaction (usually redox) where a molecule forms two or more dissimilar products. dissociation reaction - chemical reaction in which a reactant breaks into two or more parts. dissolve - a solute passing into solution, usually a solid going to the liquid phase. distillate - vapor formed by a distillation, which may be condensed into a liquid for collection. distillation - technique of heating a liquid to form a vapor, which is cooled to separate components of the liquid based on volatility or boilings. divalent cation - positive charged ion with a valence of 2. DNA - deoxyribonucleic acd, an organic molecule that codes for proteins. double bond - chemical bond in which two electron pairs are shared between two atoms. double replacement reaction - chemical reaction in which two reactants exchange anions/cations to form two new products using the same ions. dry ice - the solid form of carbon dioxide dubnium - transition metal with element symbol Db and atomic number 105. ductile - able to be stretched into a wire without breaking. dynamic equilibrium - chemical equilibrium between the forward and reverse reaction in which the rates of reaction are equal to each other. dysprosium - rare earth metal with element symbol Dy and atomic number 66. E - Effective Nuclear Charge to Extensive Property Electrons are particles with negative charge that orbit the atomic nucleus. Ian Cuming / Getty Images effective nuclear charge - net charge an electron experiences in an atom that has multiple electrons. effervescence - foaming or bubbling when gas is evolved by a liquid or solid. efflorescence - process by which a hydrate loses water of hydration. effusion - movement of gas through a pore or capillary into a vacuum or another gas. einsteinium - Einsteinium is the name for the element with atomic number 99 and is represented by the symbol Es. It is a member of the actinide group. elasticity - physical property of matter describing the ability to return to original shape after deformation. electrical conductivity - measure of a substance's ability to carry an electrical current. electrical resistivity - measure of how much a material resists carrying an electrical current. electrochemical cell - device that generates a potential difference between electrodes via chemical reactions. electrochemistry - scientific study of reactions and species formed at the interface between an electrolyte and a conductor, where electron transfer occurs. electromotive force - emf - the electric potential generated by either an en electrochemical cell or changing magnetic field. electrode - the anode or cathode of an electrical cell. electrolysis - passage of direct current through an ion-conducting solution, producing a chemical change at the electrodes. electrolyte - a substance that forms ions in aqueous solution. electrolytic cell - type of electrochemical cell in which the flow of electric energy from an external source enables a redox reaction. electromagnetic radiation - light; self propagating energy that has electric and magnetic field components. electron - stable negatively charged subatomic particle. electron affinity - measure of the ability of an atom to accept an electron. electron capture (EC) - form of radioactive decay in which the atomic nucleus absorbs a K or L shell electron, converting a proton into a neutron. electron cloud - region of negative charge surrounding the atomic nucleus that has a high probability of containing electrons. electron configuration - description of the population of the electronic energy sublevels of an atom. electron density - representation of the probability of finding an electron in a specific region around an atom or molecule. electron domain - the number of lone electron pairs or bond locations around an atom or molecule. electronegativity - property of an atom that reflects its ability to attract electrons in a chemical bond. electron pair repulsion - principle that electron pairs surrounding a central atom orient themselves as far apart as possible; used to predict geometry. electron-sea model - model of metallic bonding in which cations are described as fixed points in a mobile sea of electrons. electron spin - property of an electron related to its spin about an axis, described by a quantum number as either +1/2 or -1/2. electrophile - atom or molecule that accepts an electron pair to form a covalent bond. electroplating - process of adding a metal coat to a material by using a reduction reaction. electrostatic forces - forces between particles due to their electrostatic charges. electrum - a natural alloy of gold and silver. element - a substance that cannot be subdivided using chemical means; identified by the number of protons in its atoms. elementary reaction - chemical reaction in which reactants form products in a single step without a transition state. element symbol - the one- or two-letter abbreviation of a chemical element (e.g., H, Cl). emissions - products of a combustion reaction, aside from heat and light (e.g., carbon dioxide). emission spectrum - range of wavelengths emitted by an atom stimulated by electricity or heat. empirical formula - formula that shows the ratio of elements in a compound, but not necessarily their actual numbers in a molecule. emulsifier - stabilizing agent that prevents immiscible liquids from separating. emulsion - colloid formed from two or more immiscible liquids where one liquid contains a dispersion of the other liquid(s). enantiomer - a member of a pair of optical isomers. endothermic - process which absorbs thermal energy from its environment. enediol - an alkene enol with a hydroxyl group attached to both carbon atoms of the C=C bond. energy - the ability to do work (e.g., kinetic energy, light). enthalpy - thermodynamic property of a system that is the sum of the internal energy and the product of pressure and volume. enthalpy change - the energy change of a system at constant pressure. enthalpy of atomization - quantity of enthalpy change when chemical bonds are broken in a compound to form individual atoms. enthalpy of reaction - difference between total enthalpy of products and total enthalpy of reactants of a chemical reaction. entropy - measure of the disorder of a system. enzyme - An enzyme is a protein that functions as a catalyst for a chemical reaction. equilibrium constant - ratio of the equilibrium concentration of products raised to the power of their stoichiometric coefficients to the equilibrium concentration of the reactants raised to the power of their stoichiometric coefficients. equivalence point - point in a titration where the titrant completely neutralizes the analyte. erbium - Erbium is element atomic number 68 on the periodic table. essential amino acid - amino acid needed in the diet because an organism cannot synthesize it. ester - RCO2R′, where R is the hydrocarbon parts of the carboxylic acid and R′ is the alcohol. ether - organic compound containing two aryl or alkyl groups bound to an oxygen, R-O-R'. europium - Europium is the name for the element with atomic number 63 and is represented by the symbol Eu. It is a member of the lanthanide group. eutectic - homogeneous solid mixture of at least two types of atoms or molecules that form a superlattice (usually a mix of alloys). evaporation - process characterized by a spontaneous transition of molecules from the liquid phase to vapor phase. excess reactant - reactant left over in a reaction because it is present in a great amount than needed to react with the limiting reactant. excited state - atom, ion, molecule, or subatomic particle in a higher energy level than its ground state. exergonic - releasing energy to its surroundings. exothermic - releasing energy to the environment in the form of heat; a type of exergonic process exothermic reaction - a chemical reaction that releases heat. extensive property - property of matter that depends on the quantity of matter that is present (e.g., volume). F - F Orbital to Fusion The flame test is an analytical technique used to help identify metal ions. (c) Philip Evans / Getty Images f orbital - electron orbital with l = 3 for the angular momentum quantum number, family - a group of elements that share similar properties. Faraday constant - a physical constant equal to the electric charge of one mole of electrons, 96485.33 C/mol. fat - triesters of glycerol and fatty acids that are soluble in organic solvents, but generally insoluble in water. fatty acid - a carboxylic acid with a long hydrocarbon side chain. feedstock - any unprocessed material used as a supply for a manufacturing process. fermium - Fermium is the name for the element with atomic number 100 and is represented by the symbol Fm. It is a member of the actinide group. first law of thermodynamics - law which states the total energy of a system and its surroundings is a constant value; the law of conservation of energy. fire point - the lowest temperature a vapor will initiate and sustain combustion. fission - the splitting of an atomic nucleus, which results in two or more lighter nuclei and a release of energy. flame test - an analytical technique used to identify ions based on their emission spectrum in a flame. flammable - easily ignited or capable of sustained combustion. fluid - a substance that flows under applied shear stress, including liquids, gases, and plasma. fluorescence - luminescence released when an atom absorbs electromagnetic radiation and emits a photon when the electron falls to a lower energy state. foam - a substance containing gas bubbles trapped within a liquid or solid. force - a push or pull on a mass, with both magnitude and direction (vector). formal charge - the difference between the number of valence electrons of an atom and the number of electrons associated with the atom (e.g., in a chemical bond). formation reaction - reaction in which one mole of a product is formed. formula mass or formula weight - the sum of the atomic weights of the atoms in a compound's empirical formula. fractional distillation - process which separates components of a mixture according to their boiling points. francium - alkali metal with element symbol Fr and atomic number 87. free energy - the amount of internal energy of a system that is available to do work. free radical - an atom or molecule with an unpaired electron. freezing - process in which a liquid changes to a solid. freezing point - temperature at which a liquid transitions to a solid (not always the same as melting point). freezing point depression - lowering the freezing point of a liquid by adding another compound to it. frequency - number of times a point on a wave passes a reference point in one second. functional groups or functional moiety - group of atoms in a molecule that are responsible for characteristic reactions and properties. fusion - combining light atomic nuclei to form a heavier nucleus, accompanied by the release of energy. G - Gadolinium to Group Test tubes are a common type of chemistry glassware. Cultura Science/GIPhotoStock / Getty Images gadolinium - rare earth metal with element symbol Gd and atomic number 64. gallium - metal with element symbol Ga and atomic number 31.galvanic cell - electrochemical cell where reactions between dissimilar conductors occur through a salt bridge and electrolyte.gamma radiation - high energy ionizing photons, originating from the atomic nucleus.gas - state of matter characterized by having neither a defined shape nor defined volume.gas constant (R) - the constant in the Ideal Gas Law; R = 8.3145 J/mol·K. Gay-Lussac's law - form of the ideal gas law that states the pressure of an ideal gas is directly proportional to its absolute (Kelvin) temperature when volume is held constant.gel - a type of sol where the solid particles are held in a mesh to form a rigid or semi-rigid mixture.geometric isomer - molecules with the same number and type of atoms as each other, but with different geometrical configurations. Also called cis-trans or configurational isomerism.germanium - metalloid with element symbol Ge and atomic number 32.Gibbs free energy - a measure of the potential for reversible or maximum work done by a system at constant pressure and temperature. glass - an amorphous solid. glycosidic bond - a covalent bond between a carbohydrate and a functional group or another molecule.gold - yellow-colored transition metal with element symbol Au and atomic number 79.Graham's Law - relation stating the rate of effusion of a gas is inversely proportional to the square root of its molecular mass or density.grain alcohol - purified form of ethyl alcohol made from distilling fermented grain.gram - unit of mass equal to the mass of one cubic centimeter of water at 4°C.gram molecular mass - the mass in grams of one mole of a molecular substance. gravimetric analysis - a set of quantitative analytical techniques based on measurement of a sample's mass.green chemistry - branch of chemistry concerned with lessening the environmental effect of chemicals, including development of new materials and processes.ground state - the lowest energy state of an atom, ion, molecule, or subatomic particle. group - a vertical column on the periodic table consisting of elements that share periodic properties. H - Haber Process to Hypothesis Heat refers to thermal energy. Tim Robberts / Getty Images Haber process - method of making ammonia or fixing nitrogen by reacting nitrogen and hydrogen gas hafnium - transition metal with element symbol Hf and atomic number 72. half-cell - half of an electrolytic or voltaic cell, serving as the site of either oxidation or reduction. half-life (t1/2) - time required to convert half of the reactant to a product or the time required for half of a radioactive isotope to decay into its daughter isotope. halide ion - a singlet halogen atom, which has a charge of -1 (e.g., Cl-) halogen - an element in Group VIIA of the periodic table (e.g., Br, Cl). halogenated hydrocarbon - a hydrocarbon that contains one or more halogen atoms. hard water - water that contains high amounts of calcium and/or magnesium cations. hassium - transition metal that is atomic number 108 with element symbol Hs. heat - energy that flows between matter samples because of a temperature difference. heat capacity - quantity of heat needed to raise the temperature of a sample by a specified amount. heat of formation ( ΔHf) - amount of heat absorbed or released during formation of a pure substance from its elements at constant pressure. heat of fusion ( ΔHfus) - the change in enthalpy (heat) for the conversion of one gram or mole of a solid to a liquid at constant temperature and pressure. heavy metal - a dense metal that is toxic at low concentrations. Heisenberg uncertainty principle - principle that states it is impossible to determine both the position and momentum of a particle at once with perfect accuracy. helium - Helium is the name for the element with atomic number 2 and is represented by the symbol He. It is a member of the noble gases group. Henderson-Hasselbalch equation - an approximation that relates the pH or pOH of a solution, the pKa or pKb, and the ratio of concentration of dissociated species. Henry's Law - law that states the mass of a gas that will dissolve into solution is directly proportional to the partial pressure of the gas above the solution. Hess's Law - law that states the energy change in an overall reaction equals the sum of the energy changes in its individual (partial) reactions. heterogeneous -- consisting of dissimilar components. heterogeneous mixture - a mixture that lacks a uniform composition such that at least two components are present with identifiable properties. heterogeneous reaction - chemical reaction in which reactants are different phases from each other. holmium - rare earth metal with element symbol Ho and atomic number 67. homogeneous - uniform through its volume. homopolymer - polymer in which every mer unit is identical. hybrid orbital - orbital formed by the combination of two or more atomic orbitals. hydration reaction - reaction in which a hydrogen and hydroxyl ion are attached to a carbon in a C-C double bond. hydrocarbon - molecule consisting entirely of carbon and hydrogen atoms. hydrogen - element with atomic number 1 and symbol H. hydrogen bond - attractive interaction between a hydrogen bonded to an electronegative atom and a different electronegative atom. hydrogenation - reduction reaction that produces hydrogen (usually as H2). hydrolysis - decomposition reaction in which one reactant is water. Reverse of a condensation reaction. hydrometer - instrument used to measure the relative densities of two liquids. hydronium ion - the H3O+cation. hydrophobic - property of repelling water. hydroxyl group - functional group consisting of a hydrogen atom covalently bonded to an oxygen atom (-OH). hygroscopic - able to absorb or adsorb water from the surroundings. hypertonic - having higher osmotic pressure than another solution. hypothesis - prediction of an event or proposed explanation of a phenomenon. I - Ideal Gas to IUPAC Fluids that don't mix are said to be immiscible. Greg Samborski / Getty Images ideal gas - gas in which molecules have negligible size and kinetic energy dependent only on temperature. ideal gas constant - physical constant in the Ideal Gas Law, equal to the Boltzmann constant but with different units. ideal gas law - PV = nRT where P is pressure, V is volume, n is number of moles, R is the ideal gas constant, and T is temperature. immiscible - property of two substances being unable to combine to form a homogeneous mixture; unable to mix independent variable - the variable that is controlled or changed in an experiment to test its effect on the dependent variable. indicator - substance that undergoes a visible change when its conditions change (e.g., a pH indicator). indium - metal with element symbol In and atomic number 49. inductive effect - effect a chemical bond has on the orientation of adjacent bonds in a molecule. inhibitor - substance that slows or prevents a chemical reaction. inorganic chemistry - study of chemistry of molecules of non-biological origin (not containing C-H bonds). insoluble - unable to dissolve in a solvent. intensive property - property of matter that is independent of the quantity of matter in a sample. intermolecular force - the sum of all forces between neighboring molecules. internal energy - the total energy (U) of a closed system. intrinsic property - property of matter that is independent of the quantity of matter present. intermediate - substance formed in a middle step between reactants and final products. inverse proportion - relationship between variable such that their product is a constant value. iodine - Iodine is the name for the element with atomic number 53 and is represented by the symbol I. It is a member of the halogen group. ion - atom or molecule which has a different number of protons than electrons and thus a net electrical charge. ionic - pertaining to carrying a net electrical charge at the atomic or molecular level. ionic bond - chemical link between atoms caused by electrostatic force between opposite charged ions. ionic compound - compound formed by ions bonding together due to electrostatic forces (differing electronegativity values). ionic equation - chemical equation in which electrolytes in aqueous solution are written as dissociated ions. ionic radius - half the distance between two ions just touching each other. ionization energy - energy required to completely remove an electron from a gaseous atom of ion. iridium - Iridium is the name for the element with atomic number 77 and is represented by the symbol Ir. It is a member of the transition metals group. iron - Iron is the name for the element with atomic number 26 and is represented by the symbol Fe. It is a member of the transition metals group. isoelectronic - chemical species that have the same electronic structure and thus same number of valence electrons. isolated system - thermodynamic system that can't exchange energy or matter outside of the system. isomer - chemical species with the same number and type of atoms as another species, but a different arrangement and thus different properties. isomerization process - protocol in which straight chain hydrocarbons are converted into branched chain hydrocarbons. isotopes - atoms that have the same number of protons, but different numbers of neutrons and thus different atomic weight values. IUPAC - International Union of Pure and Applied Chemistry, an authority on chemical standards. Chemistry Definitions Starting With the Letter J The Joule is a unit of energy. Paper Boat Creative / Getty Images joule - SI unit of energy equal to the kinetic energy of a 1 kg mass moving at 1 m/s. K - Kelvin Temperature to Krypton Krypton is a noble gas. Science Picture Co / Getty Images Kelvin temperature scale - an absolute temperature scale with 100 degrees between the freezing and boiling points of water (although values are given without degrees by convention). keratin - a fibrous protein produced by chordates. It may be found in hair, skin, claws, and wool. ketone - compound containing a carbonyl functional group (C=O) between two groups of atoms kilo - prefix meaning "one thousand". kilopascal (kPa) - unit of pressure exerted by a 10 g mass on a square centimeter. There are 1000 Pa in 1 kPa. kinetic energy - energy associated with motion. krypton - element 36 on the periodic table with symbol Kr. L - Labile Complex to Lutetium Litmus paper is a specific type of pH paper. Clive Streeter / Getty Images labile complex - a complex ion that quickly reaches equilibrium with ligands in the surrounding solution. lanthanides - subset of transition metals characterized by filling of the 4f sublevel, usually atomic number 58-71. lanthanum - element atomic number 57 with element symbol La. lattice energy - enthalpy change of the process by which opposite-charged ions in a gas combine to form a solid ionic lattice. law - a general rule that explains a body of scientific observations. Laws are stated in words, but expressed by mathematical equations. Law of Chemical Equilibrium - an expression of the relationship between the concentration of reactants and products of a chemical reaction mixture at equilibrium. Law of Combining Volumes - relation that states the volumes of gases in a chemical reaction are present in the ratio of small integers under conditions where all gases are at the same temperature and pressure. Law of Conservation of Energy - law that states energy can neither be created nor destroyed, although it may change from one form into another. Law of Conservation of Mass - law that states matter in a closed system may be neither created nor destroyed, although it may change forms. Law of Constant Composition - chemistry law that states samples of a pure compound contain the same elements in the same proportions by mass. Law of Definite Proportions - law that states all samples of a compound contain the same proportion of elements by mass. Law of Multiple Proportions - law that states element combine in ratios of small whole numbers to form molecules. lawrencium - actinide with element symbol Lr and atomic number 103. lead - metal with element symbol Pb and atomic number 82. Le Chatelier′s Principle - principle that says that equilibrium of a chemical system will shift in the direction to relieve stress. Lewis acid - chemical species that can act as an electron pair acceptor. Lewis base - a substance that is an electron pair donor. Lewis acid base reaction - chemical reaction that forms at least one covalent bond between an electron pair donor (Lewis base) and electron pair acceptor (Lewis acid). Lewis structure - representation of a molecule that uses dots to show electrons around atoms and lines to show covalent bonds. ligand - a chemical species that donates or shares at least one electron via a covalent bond with a central ion or atom. limiting reactant - the reactant that determines how much product may result from a chemical reaction. lipid - class of fat-soluble molecules, also known as oils and fats liquefaction - process of converting a material from a solid or gas phase into the liquid phase. liquid - state of matter characterized by having a definite volume but not a definite shape. lithium - alkali metal with atomic number 3 and element symbol Li. litmus paper - filter paper used as a pH paper that has been treated with a water-soluble dye obtained from lichens. London dispersion force - weak intermolecular force between atoms or molecules in close proximity to each other, due to electron repulsion. lone pair - an electron pair in the outer shell of an atom that is not shared or bonded with another atom. lutetium - rare earth metal with element symbol Lu and atomic number 71. M - Macromolecule to Muriatic Acid Mass is a measure of the quantity of matter in a sample. Larry Washburn / Getty Images macromolecule - molecule containing a very large number of atoms, usually more than 100. Madelung's rule - rule that describes filling of electron orbitals in atoms due to shielding of nuclear charge by inner electrons. magnesium - Magnesium is the name for the element with atomic number 12 and is represented by the symbol Mg. Magnesium is an alkaline earth metal. main group elements - any of the elements in the s and p blocks of the periodic table. malleable - able to be shaped or pounded with a hammer, usually applied to metals. manganese - element with atomic number 25 and element symbol Mn. manometer - device used to measure gas pressure. mass - amount of matter a substance contains or property of matter that resists acceleration. mass defect - difference between the mass of an atom and the sum of the masses of its protons, neutrons, and electrons. mass number - whole number integer that is the sum of the number of protons and neutrons in the atomic nucleus. mass percentage - concentration calculated as mass of a component divided by total mass of mixture or solution; w/w%. mass spectroscopy - analytical technique used to separate and/or identify components of a mixture based on mass and electrical charge. matter - anything that has mass and occupies volume. measurement - quantitative or numerical data describing an object or event. medicinal chemistry - branch of chemistry concerned with design, synthesis, and study of pharmaceuticals. meitnerium - radioactive transition metal with element symbol Mt and atomic number 109. melting - phase change of matter from solid to liquid. melting point - temperature at which the solid and liquid phase of matter coexist in equilibrium. mendelevium - actinide with atomic number 101 and element symbol Md. meniscus - phase boundary between a liquid in a container and a gas, curved due to surface tension. mercaptan - organic sulfur compound containing an alkyl or aryl group and a thiol group. mercapto group - functional group consisting of a sulfur bonded to a hydrogen; -SH. mercury - transition metal with element symbol Hg and atomic number Hg. metabolism - set of biochemical reactions that store chemical energy and convert it into a form an organism can use. metal - substance that has high conductivity and other metallic properties, including tendency to form cations, often identified by group on the periodic table. metallic character - set of chemical properties associated with metals, including the ability to lose outer valence electrons to form cations. metallic compound - chemical compound that contains one or more metal atoms. metalloid - element with properties intermediate between those of metals and nonmetals (e.g., silicon). meter - either (a) the base unit of length in the SI system or (b) a device used to measure a quantity. methyl - functional group containing a carbon bonded to three hydrogen atoms, -CH3. microliter - unit of volume that is one-millionth of a liter one cubic millimeter. micron - unit of length equal to one-millionth of a meter; a micrometer. mineral acid - any inorganic acid (e.g., sulfuric acid). miscible - soluble or able to be mixed to form a solution, typically applied to fluids. mixture - combination of two or more substances such that each retains its separate chemical identity (e.g., salt and flour). moderator - material that slows or moderates the speed of neutrons. Mohs scale - Mohs scale is a relative scale rating the hardness of a mineral. A mineral with a high Mohs number is able to mark a mineral with a lower Mohs number. moiety - group of atoms in a molecule that are responsible for its characteristic chemical behavior. molality - unit of concentration that is the moles of solute divided by the kilograms of solvent. molar - refers to molarity (moles per liter of solution); e.g. a 6 M HCl solution has 6 moles of hydrochloric acid per liter of solution. molar enthalpy of fusion - energy needed to change one mole of a substance from solid to liquid phase at constant pressure and temperature. molar enthalpy of vaporization - energy needed to change one mole of liquid to the gas phase at constant pressure and temperature. molarity - unit of concentration that is the number of moles of solute divided by the number of liters of solution. molar mass - mass of one mole of a substance. molar heat capacity - heat energy required to raise the temperature of 1 mole of a substance 1 Kelvin. molar volume - volume of one mole of a substance. mole - chemical mass unit equal to 6.022 x 1023 molecules, atoms, or other particles. molecular equation - balanced chemical equation in which ionic compounds are expressed as molecules rather than ions. molecular formula - expression of the number and type of atoms in a molecule. molecular geometry - description of the shape of a molecule and the relative positions of its atoms. molecular mass - sum of the atomic masses of atoms in a molecule. molecular orbital - wave function of an electron in a molecule. molecular weight - sum of the atomic weights of atoms in a molecule. molecule - chemical species formed by two or more atoms that share chemical bonds such that they form one unit. mole fraction - unit of concentration that is the number of moles of a component divided by the total number of moles of a solution. mole ratio - ratio or fraction comparing the numbers of moles of any two components involved in a chemical reaction. molybdenum - transition metal with element symbol Mo and atomic number 42. monatomic ion - an ion formed by a single atom. monomer - a molecule that is a subunit or building block of a polymer. monoprotic acid - acid that donates a single proton or hydrogen atom per molecule in aqueous solution. mother liquor - solution remaining after crystals are removed from a crystallization solution. MSDS - acronym for Material Safety Data Sheet, a written document outlining safety information about a chemical. multiple bond - a bond formed when two or more pairs of electrons are shared between two atoms. muriatic acid - common name for hydrochloric acid, HCl. N - Napthenes to Nutraceutical Neon lights contain the noble gas neon. Jill Tindall / Getty Images naphthenes - cyclic aliphatic hydrocarbons from petroleum with the general formula CnH2n. natural abundance - average percentage of a given isotope naturally occurring on Earth. neodymium - rare earth metal with element symbol Nd and atomic number 60. neon - noble gas with element symbol Ne and atomic number 10. neptunium - actinide with element symbol Np and atomic number 94. net ionic equation - chemical equation that lists only the species participating in the reaction. network solid - material consisting of an array of repeating covalently bonded atoms. neutral solution - aqueous solution with a pH of 7. neutralization - chemical reaction between an acid and base that results in a neutral solution. neutron - particle in the atomic nucleus that has a mass of 1 and charge of 0. newton (N) - SI unit of force equal to the amount of force needed to accelerate a 1 kg mass 1 m/sec2. nickel - Nickel is the name for the element with atomic number 28 and is represented by the symbol Ni. Nickel is a member of the transition metals group. niobium - Niobium is the name for the element with atomic number 41 and is represented by the symbol Nb. Niobium is also called Columbium and is a transition metal. nitrogen - Nitrogen is the name for the element with atomic number 7 and is represented by the symbol N. Nitrogen is also known as azote and is a member of the nonmetal group. nobelium - actinide with element symbol No and atomic number 102. noble gas - element from Group 8 of the periodic table (e.g., xenon, argon). noble gas core - shorthand notation used writing atomic electron configuration in which previous noble gas configuration is replaced by the element symbol in brackets. nonbonding electron - electron in an atom that does not participate in a chemical bond with other atoms. nonelectrolyte - substance that does not dissociate into ions in aqueous solution. nonmetal - element that does not display metallic properties, typically referring to elements located in the upper right corner of the periodic table. nonoxidizing acid - an acid that cannot act as an oxidizing agent. nonpolar bond - chemical bond with even distribution of charge such that it does not have positive or negative poles. nonpolar molecule - molecule that has even distribution of charge such that it does not have positive and negative sides. nonspontaneous reaction - chemical reaction that cannot occur without input of external work. nonvolatile - substance that does not readily evaporate into a gas under ordinary conditions. normal boiling point - temperature at which a liquid boils at 1 atm of pressure (sea level). normal concentration - either refers to normal concentration in which the concentration of solutes is the same in two samples or refers to gram equivalent weight of a solute in solution (N). normality (N) - measure of concentration equal to gram equivalent weight per liter of solution. normal melting point - temperature at which a solid melts at 1 atm of pressure. nuclear fission - splitting of atomic nuclei into two or more lighter nuclei, accompanied by an energy release. nuclear radiation - particles and photons emitted during reactions in the atomic nucleus. nucleation - process of vapor droplets condensing into a liquid, bubbles forming in a boiling liquid, or particle accretion to grow crystals. nucleophile - atom or molecule that donates an electron pair to form a covalent bond. nucleotide - organic molecule comprised of a nucleotide base, ribose or deoxyribose, and one or more phosphate groups. nucleus - positively charged center of an atom, made from protons and neutrons. nuclide - an atom or ion characterized by the proton and neutron composition of its nucleus. null hypothesis - proposition that there is no effect of a treatment or no relationship between an independent and dependent variable. nutraceutical - a food or part of food that confers health or medical benefits. O - Octane Number to Oxygen Two oxygen atoms bond to form an oxygen molecule. ADAM HART-DAVIS/SCIENCE PHOTO LIBRARY / Getty Images octane number - value that indicates resistance of motor fuel to engine knock relative to the knock from isooctane (100) and heptane (0). octet - group of 8 valence electrons around an atom. octet rule - principal that atoms in an atomic bond share their 8 outer electrons. open system - a system able to freely exchange matter and energy with its surroundings. orbital - mathematical function that describes the wavelike behavior of an electron. organic chemistry - study of the chemistry of compounds containing carbon chemical bonded to hydrogen. osmium - Osmium is the name for the element with atomic number 76 and is represented by the symbol Os. It is a member of the transition metal group. osmosis - movement of solvent molecules across a semipermeable membrane from a dilute solution to a more concentrated solution, thus diluting it and equalizing concentration on both sides of the membrane. oxidant - a reactant that oxidizes or removes electrons from another reactant in a redox reaction. oxidation - loss of electrons by an atom, molecule or ion in a chemical reaction. oxidation number - the electrical charge of a central atom in a coordination compound if all electron pairs and ligands were removed. oxidation state - the difference between the number of electrons in an atom in a compound compared with the number of electrons in a neutral atom of the element.