Gas is one of the four fundamental states of matter, found between the liquid and plasma states, where their particles are widely separated and have very weak intermolecular bonds between the atoms. Gases are the only states of matter that can be compressed or even expand to fill a very large space. It is these properties which makes them subject to behave differently with respect to changing physical properties such as pressure, volume, temperature and amount.
In the periodic table, gases tend to be the elements which are extremely light such as hydrogen or helium, or other elements which can form molecules with no strong dipole (assuming standard temperature or pressure). For example N2 and O2 are gases typically seen in Earth’s atmosphere, with 79% and 21% composition respectively. Other non-reactive gases include Group 18 of the periodic table known as the Noble Gas Group. They are non-reactive because they already have a full octet of electrons, and thus do not need to gain or lose any electrons to be electrically stable. An example of a noble gas is Argon, which is used in incandescent lights due to their unreactive nature to prevent the oxidation of the filaments as a result of the very high temperature.
Thus, understanding the properties of gases is important to understand and interpret any gaseous reactions.