Gas Definition and Examples (Chemistry)

Chemistry Glossary Definition of Gas

Water vapor is the gas state of water.
Water vapor is the gas state of water. Bryan Mullennix / Getty Images

Gas Definition

A gas is defined as a s​tate 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:

List of the Elemental Gases

There are 11 elemental gases (12 if you count ozone). Five are homonuclear molecules, while six are monatomic:

  • H2 - hydrogen
  • N2 - nitrogen
  • O2 - oxygen (plus O3 is ozone)
  • F2 - fluorine
  • Cl2 - chlorine
  • He - helium
  • Ne - neon
  • Ar - argon
  • Kr - krypton
  • Xe - xenon
  • Rn - 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.