Science, Tech, Math › Science Gas Definition and Examples in Chemistry Chemistry Glossary Definition of Gas Share Flipboard Email Print Water vapor is the gas state of water. Bryan Mullennix / Getty Images Science Chemistry Chemical Laws Basics 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. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College 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. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated May 04, 2019 A gas is defined as a state of matter consisting of particles that have neither a defined volume nor defined shape. It is one of the four fundamental states of matter, along with solids, liquids, and plasma. Under ordinary conditions, the gas state is between the liquid and plasma states. A gas may consist of atoms of one element (e.g., H2, Ar) or of compounds (e.g., HCl, CO2) or mixtures (e.g., air, natural gas). Examples of Gases Whether or not a substance is a gas depends on its temperature and pressure. Examples of gases at standard temperature and pressure include: air (a mixture of gases)chlorine at room temperature and pressureozoneoxygenhydrogenwater vapor or steam List of the Elemental Gases There are 11 elemental gases (12 if you count ozone). Five are homonuclear molecules, while six are monatomic: H2 - hydrogenN2 - nitrogenO2 - oxygen (plus O3 is ozone)F2 - fluorineCl2 - chlorineHe - heliumNe - neonAr - argonKr - kryptonXe - xenonRn - radon Except for hydrogen, which is at the top left side of the periodic table, elemental gases are on the right side of the table. Properties of Gases Particles in a gas are widely separated from each other. At low temperature and ordinary pressure, they resemble an "ideal gas" in which the interaction between the particles is negligible and collisions between them are completely elastic. At higher pressures, intermolecular bonds between gas particles have a greater effect on the properties. Because of the space between atoms or molecules, most gases are transparent. A few are faintly colored, such as chlorine and fluorine. Gases tend not to react as much as other states of matter to electric and gravitational fields. Compared with liquids and solids, gases have low viscosity and low density. Origin of the Word "Gas" The word "gas" was coined by 17th-century Flemish chemist J.B. van Helmont. There are two theories about the origin of the word. One is that it is Helmont's phonetic transcription of the Greek word Chaos, with the g in Dutch pronounced like the ch in chaos. Paracelsus's alchemical use of "chaos" referred to rarified water. The other theory is that van Helmont took the word from geist or gahst, which means spirit or ghost. Gas vs Plasma A gas may contain electrically charged atoms or molecules called ions. In fact, it's common for regions of a gas to contain random, transient charged regions because of van der Waals forces. Ions of like charge repel each other, while ions of opposite charge attract each other. If the fluid consists entirely of charged particles or if the particles are permanently charged, the state of matter is a plasma rather than a gas.