Chemistry Glossary Terms - Definitions Starting with the Letter A

Alphabetical Chemistry Glossary and Dictionary

A is for atom, the basic chemical building block of an element.
A is for atom, the basic chemical building block of an element. Getty Images

This comprehensive chemistry glossary offers definitions for terms which are commonly used in chemistry and chemical engineering.


α-carbon - An alpha carbon is the carbon in a molecule bonded to an atom or moiety of interest. α-carbon is the most common notation for the alpha carbon.

α-hydrogen - a hydrogen atom bonded to the α-carbon in a molecule.

 α-hydrogen is the most common notation for alpha hydrogen.

Abegg's rule -Abegg's rule states the sum of the absolute values of the maximum positive and negative valence of an atom is often equal to eight.

abhesive - a material that prevents two surfaces from sticking together.

Antonym: adhesive

Examples: Teflon is an abhesive material used to make frying pans non-stick.

ab initio -Ab initio is Latin for "from first principles". Ab initio refers to predictions or calculations which are based entirely on theory as opposed to experimentation.

ablation -the process of transferring heat by removing material by melting, vaporization or other erosive process.


absolute alcohol

absolute error

absolute pressure - the gauge pressure plus atmospheric pressure.

absolute temperature

absolute uncertainty

absolute vacuum - a volume of space containing no matter. Also Known As: perfect vacuum

absolute zero - the lowest possible state at which matter can exist, 0 K or -273.15°C.


absorbed dose - the amount of energy from radiation deposited or absorbed by an object per unit mass. The SI unit of absorbed dose is the Gray (Gy) or J/kg.


absorption cross section

absorption spectroscopy

absorption spectrum

absorptivity - the absorption cross section or extinction coefficient.

Absorptivity varies with wavelength and is defined as the absorbance of a solution per unit path length and concentration:

a = A/(bc)

where a is absorptivity, A is absorbance, b is path length, and c is the concentration

abstraction - a chemical reaction where an atom or ion is removed from one molecule by a radical.

Hydrogen abstraction is different from deprotonization. In abstraction, the hydrogen atom supplies an electron to the bonding where deprotonization, the base supplies both electrons in the bond.

Example: Hydrogen abstraction of acetone by chlorine radical.

CH4 + Cl- → H3C- + HCl

abundance ratio -  the ratio of the number of atoms of one isotope to the number of atoms of another isotope in the same sample.

AC or A/C - refers to alternating current. AC or alternating current is an electric current that reverses its direction at regular intervals.

accelerator - a substance that speeds up (accelerates) a chemical reaction. The term often is applied to polymerization. An accelerator can speed the vulcanization of rubber or cause it to occur at a lower temperature than normal. More generally, an accelerator can speed cross-linking of polymer subunits or cause polymerization to occur at a lower temperature than normal.


acetal - an organic molecule where two separate oxygen atoms are single bonded to a central carbon atom. Acetals have the general structure of R2C(OR')2.

An older definition of acetal had one at least one R group as a derivative of an aldehyde where R = H, but an acetal can contain derivatives of ketones where neither R group is a hydrogen. This type of acetal is called a ketal.

Acetals that contain different R' groups are called mixed acetals. Acetal is also a common name for the compound 1,1-diethoxyethane. Dimethoxymethane is an example of an acetal compound.

acetate - 1. acetate ion (CH3COO-, C2H3O2-) an ion formed by removing the acidic hydrogen from acetic acid.
2. a fiber made from cellulose acetate.
3. a compound resulting from replacing the acidic hydrogen in acetic acid.

achiral - Achiral literally means "not chiral".

Achiral refers to an object which can be superimposed on its mirror image. Synonym: amphichiral

Methane is an example of an achiral molecule. The image shows methane's structure and its mirrored reflection. The reflection can be rotated 180° to match up with the original structure.


acid anhydride

acid-base indicator

acid-base titration

acid dissociation constant - Ka

acid catalysis -is a chemical reaction that requires the presence of an acid to act as a catalyst in order to proceed. The acid catalyst typically acts as a supply of protons to activate bonding sites in a molecule to induce a reaction.

acidic solution

acid promoted -refers to a chemical reaction that needs an acid to proceed but does not act as a catalyst for the reaction. Reactions where the acid acts as a catalyst are acid catalysis reactions.

acidulant -a food additive that lowers the pH to give a tart or bitter taste.

acology - is the study of medical remedies.

acrylic fiber - a polymer that contains at least 85% by mass acrylonitrile monomers. Also Known As: acrylic

actinic -refers to light capable of initiating a chemical reaction.

Example: Ultraviolet light is actinic since it can cause photosynthesis.

actinides - Usually, the actinides are considered to be elements 90 (thorium) through 103 (lawrencium). Otherwise, the actinides are defined according to their common properties.

Examples: thorium, uranium, plutonium

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 carbon or activated charcoal - a processed form of carbon treated to be more porous. 

Example: Activated charcoal is used as an absorbent material to remove impurities in water.

activated complex

activation energy - Ea

active ingredient - a chemical or substance that has a biological effect. The term is applied to drugs, pesticides, herbicides, and herbal medicine. The other ingredients are termed excipients or inert ingredients. Excipients are either biologically nonreactive or else do not affect the biochemical process that is targeted by the product.

A formulation may contain more than one active ingredient.

Also Known As: AI, Active Pharmaceutical Ingredient (API), bulk active, active substance, pharmacon, pharmakon

active transport

activity series

actual yield

acute health effect

acylating agent - a compound that supplies the acyl group in an acylation reaction.

acylation - a chemical reaction that adds an acyl group to a molecule. Synonym: alkanoylation

acyl group

acyl halide - a functional group with formula R-COX where X is a halogen atom.

Acyl halide also refers to organic compounds containing the acyl halide functional group.

Acyl halides are formed by substituting a hydroxyl group of an oxoacid with a halogen atom.

addition compound -a compound containing two or more simple compounds combined into an orderly defined crystal matrix. The two simple compounds are separated by a dot (·) in their formula.

addition polymer - a polymer produced through the reaction of a monomer adding to itself. No further product is formed. The monomer is most commonly a derivative of ethylene.

addition reaction - the insertion of a small molecule (e.g., H2) directly into a double or triple carbon bond.

adenosine triphosphate - ATP is the acronym for the molecule adenosine triphosphate. The empirical formula of ATP is C10H16N5O13P3.

ATP is a nucleoside triphosphate made by bonding three phosphate groups to adenosine (adenine ring plus a ribose sugar). This organic compound often is termed the 'energy currency' of cellular metabolism because hydrolysis of the phosphate bonds releases considerable energy.

In addition to its function for intracellular energy transport, ATP serves as a substrate for enzymes that produce cyclic AMP (adenosine monophosphate) and phosphorylate lipids and proteins.

adhesive - a material which bonds together the surfaces of two other materials. Examples: glues and cements



aeration - a process where air is forced into a substance. Example: Aquarium bubblers increase the oxygen content of the water by aeration.




ala - an abbreviation for the amino acid alanine. Alanine is also abbreviated as A.

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.


alcoholate - alcoholate can refer to either alkoxide anions or salts where alcohol replaces the water of crystallization in hydrated crystals.

alcohol dehydrogenase -an enzyme that facilitates reactions to oxidizes alcohol to an aldehyde or ketone in conjunction with NADH. In reverse, alcohol dehydrogenases assist to reduce aldehydes and ketones to alcohols.

alcoholysis - a substitution chemical reaction which has an alcohol as a reactant that is incorporated into part of the product molecule. Alcoholysis reactions have the form:

Alcohol + R-LG → R-Alcohol + LG

where LG is a leaving group.

aldehyde - an organic compound containing the -CHO functional group at the end of a hydrocarbon chain. Aldehyde also refers to the aldehyde functional group R-CHO appearing at the endpoints of molecules. 

aldoheptose - a heptose carbohydrate with an aldehyde functional group at the first carbon.

aldohexose -a hexose carbohydrate with an aldehyde functional group at the first carbon.

aldoxime -an oxime where one R group is a hydrogen.

aldopentose - a pentose carbohydrate with an aldehyde functional group at the first carbon.

aldose - a molecule made up of a monosaccharide bonded to an aldehyde chain. Aldose molecules have a general chemical formula of Cn(H2O)n.

aldotetrose -a tetrose carbohydrate with an aldehyde functional group at the first carbon.

algaecide or algicide - a substance used to control or kill algae. Example: Copper sulfate is used as an algaecide in aquariums and ornamental ponds.

aliphatic - refers to organic molecules or functional groups where the carbon bonds are not aromatic. Examples: All simple alkane chains are aliphatic.

aliphatic amino acid

aliphatic compound

aliphatic group - a functional group where the group is made up of carbon and hydrogen atoms and not aromatic. Example: The propyl functional group is an aliphatic group.

aliphatic hydrocarbon

alkali metal


alkaline earth metal - an element belonging to group 2 of the periodic table. The alkaline earth metals, as a group, share characteristic properties.

Alternate Spellings: alkaline earths

Examples: beryllium, magnesium, calcium, barium.


alkaloid - a class of organic compounds with at least one nitrogen in a heterocyclic ring. Alkaloids often have pharmacological effects on humans.

Examples of alkaloids: caffiene, nicotine, morphine, cocaine, theobromine

alkane - a hydrocarbon containing only single carbon-carbon bonds.

alkanoylation - a chemical reaction that adds an acyl group to a molecule. Synonym: acylation


alkenyl group


alkoxy group

alkylammonium salt - an ammonium salt where the ammonium cation has the general structure NRxH4-x where x = 1-4. Dimethylamine hydrochloride is an alkylammonium salt. It is also known by the name dimethyl ammonium chloride.

alkylate - the product formed by the reaction of an alkane and an alkyne.

alkylation - the the process where an alkyl group is introduced into a molecule.

alkyl group - a hydrocarbon group, such as CH3- or C3H7-.

alkyne - a hydrocarbon containing a triple carbon-carbon bond. Example: Acetylene



alpha carbon - the carbon in a molecule bonded to an atom or moiety of interest. α-carbon is the most common notation for the alpha carbon.

alpha decay

alpha hydrogen - a hydrogen atom bonded to the α-carbon in a molecule. α-hydrogen is the most common notation for alpha hydrogen.

alpha particle - a He2+ ion or the helium nucleus. This particle is commonly denoted by the Greek letter α.

alpha radiation

alternating copolymer - a type of polymer consisting of two different repeating mer units in which the mer units alternate positions within the chain of the molecule.

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.



amidogen - a radical composed of a nitrogen and two hydrogen atoms (NH2)

Synonyms: amino radical (preferred IUPAC name), amido, azanyl


amine functional group - a functional group containing three substituents around a central nitrogen atom containing a lone pair of electrons.

Amines are further classified by the number of substituents replaced by hydrogen. Primary amines have two substituents replaced by hydrogen. Secondary amines have one substituent replaced by hydrogen. Tertiary amines have no hydrogen substituents. Ammonia is formed when all three substituents are hydrogen.

amino acid

aminolysis - Aminolysis is a substitution chemical reaction which has an amine as a reactant that is incorporated into part of the product molecule. Aminolysis reactions have the form:
Amine + R-LG → R-Amine + LG
where LG is a leaving group.

ammeter - An ammeter (or ampmeter) is an instrument used to measure current.

ammonium - Ammonium is a cation with formula NH4+. It is the conjugate acid of ammonia. Ammonium is also added into the name of any molecule where a nitrogen atom has four single bonds and positive formal charge.

ammonium salt - An ammonium salt is a salt containing an ammonium cation and any anion. Ammonium nitrate and ammonium chloride are both ammonium salts.

amorphography - Amorphography is the science concerned with the classification and characterization of amorphous solids.



amphetamine - An amphetamine is a psychostimulant drug, based on the chemical formula C9H13N. Amphetamine is contraction of alphamethyl-phenylethylamine, which has the systematic name 1-phenylpropan-2-amine. It belongs to the phenethylamine class of molecules. Derivatives of amphetamines, such as dextroamphetamine or the sulfate or phosphate of amphetamine, also are considered to be amphetamines. Pure amphetamine is a pure colorless liquid.



amphoteric oxide

amplitude - Amplitude refers to the magnitude of change in an oscillating system. Peak to peak amplitude refers to the total change between maximum and minimum values of the oscillating system. Semi-amplitude is half the peak to peak amplitude. In general use, the term amplitude refers to the semi-amplitude.


anaerobic - Anaerobic means "without oxygen".

analyte - Analyte is the substance being analyzed in a analytical procedure.

analytical chemistry


angular momentum quantum number



androgen - Androgen is the name given to any natural or synthetic compound that stimulates or controls male sex characteristics. Androgens typically are steroid hormones. Androgens are precursor molecules to estrogens, the female sex hormones.
Examples: testosterone, dihydroxytestosterone

anisotropy - Anisotropy refers to a material exhibiting different values of a property in different crystallographic directions.

annealing - Annealing is a term used to denote any heat treatment in which the microstructrure and therefore the properties of a material are altered. Annealing typically refers to heat treatment in which a cold-worked metal is softened by allowing it to recrystallize.


anodize - Anodize refers to coating a metal with a protective layer by means of electrolysis.

anti addition - Anti addition is an addition reaction that adds two substituents to opposite sides of a double or triple bond such that the bond order of the bond decreases but the number of substituents increases.

antiarmomaticity - Antiaromaticity refers to a planar ring molecules with 4n conjugated delocalized π-electrons in the rings where n is an integer. Antiaromatic molecules are unstable and highly reactive. Antiaromaticity differs from aromaticity by the number of π-electrons. Antiaromaticity has 4n, aromaticity has 4n+2.

antibonding orbital

antichlor - An antichlor is a substance that removes excess chlorine to stop a bleaching reaction.
Example: Sodium bisulfate and other trisulfates are antichlors.

antiferromagnetism - Antiferromagnetism refer to a phenomenon exhibited by some materials in which complete magnetic moment cancellation occurs as a result of antiparallel coupling of adjacent atoms or ions. The macroscopic solid of an antiferromagnetic material has no net magnetic moment.

anti-Markovnikov addition

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.

antioxidant - An antioxidant is defined as an enzyme or other organic molecule that can counteract the damaging effects of oxygen in tissues. Although the term technically applies to molecules reacting with oxygen, it is often applied to molecules that protect from any free radical (molecules with unpaired electron).
Examples: beta-carotene, lycopene, vitamin E


aprotic solvent - An aprotic solvent is a solvent that does not does not donate hydrogen (or proton).
Example: Acetone is an aprotic solvent. 


aqueous solution

aqua fortis - Aqua fortis is an old name for nitric acid (HNO3).
Also Known As: acid of nitre, acid of spirit, spirit of nitre

aqua regia

aqua vitae - Aqua vitae or aqua vita is an old word for a concentrated solution of ethanol in water or strong spirits.

arene - An arene is a aromatic hydrocarbon molecule. Benzene is a simple arene. Arenes are also called simply aromatic hydrocarbons or aryl hydrocarbons.

arg - Arg is an abbreviation for the amino acid arginine. Arginine is also abbreviated as R.

argentum - Argentum is the Latin name for the element silver. Silver's symbol Ag comes from argentum.

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

Arrhenius acid

Arrhenius base

Arrhenius rate equation - The Arrhenius rate equation is a mathematical expression which relates the rate constant of a chemical reaction to the exponential value of the temperature.



aryl halide - An aryl halide is a molecule where a halogen atom is bonded to a carbon atom that is part of an aryl ring.
Also Known As: haloarene, halogenoarene
Examples: Chlorobenzene, fluorobenzene and bromobenzene are all aryl halide molecules.

asbestos - Asbestos is the general name for a class of materials comprised of silicate fibers. The material is known for its resistance to heat, electrical resistance and chemical inertness. The use of asbestos was reduced when it was discovered it was linked to lung disorders and lung cancer.

asn - Asn is an abbreviation for the amino acid asparagine. Asparagine is also abbreviated as N.

asp - Asp is an abbreviation for the amino acid aspartic acid. Aspartic acid is also abbreviated as D.

asphalt or asphaltic - Asphalt is a brownish-black semisolid or solid mixture of bitumens, either from a native source or as a petroleum by-product. Sometimes the term asphalt refers to a mixture of asphalt with sand, gravel or crushed stone.
Also Known As: asphaltic (adjective)
Common Misspellings: aphsalt

asphyxiant - An asphyxiant is a gas or vapor that can displace or dilute air. Asphyxiants can cause unconsciousness and/or death if inhaled.
Examples: hydrogen gas, helium gas, propane, carbon dioxide are all asphyxiants.

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.

astrochemistry - Astrochemistry is the chemistry of outer space. It is usually applied to regions beyond the solar system (which is sometimes termed cosmochemistry). Astrochemistry is an integration of astronomy and chemistry.

atactic - Atactic refers to a polymer chain configuration in which the side groups are poitioned randomly on one or the other side of the polymer backbone.


atomic ion - An atomic ion is an atom which has gained or lost at least one electron resulting in a net positive or negative charge on the atom.
Example: The hydride ion, H- is an atomic ion

atomic mass

atomic mass unit (amu)

atomic number

atomic radius

atomic solid

atomic volume

atomic weight


ATP - ATP is the acronym for the molecule adenosine triphosphate. The empirical formula of ATP is C10H16N5O13P3.

atto - Atto is a decimal prefix for SI units equal to 10-18. The symbol for atto is a.
Example: 100,000 carbon atoms weighs approximately 20 attograms.

Aufbau principle

aurum - Aurum is the Latin name for the element gold. Gold's symbol Au comes from the name aurum.


autoionization - Autoionization is an ionization reaction which occurs between identical molecules.

Avogadro's Law

Avogadro's number


azide - An azide is an anion with the molecular formula N3-. The azide functional group has the general molecular formula of RN3.
Also Known As: atisine

azimuthal quantum number

azo compound

azo group - The azo group is a functional group consisting of a diazene molecule (HN=NH) where the hydrogen atoms are replaced with alkyl or aryl groups. The general formula for an azo group is R-N=N-R'.
Also Known As: diimide functional group