A monoatomic gas is a gaseous substance composed of molecules that each contain only one atom each rather than molecules formed by the bonding of two or more atoms. Examples of monoatomic gases are helium, argon and neon.
The noble gases, found in the far right group on the periodic table, exist in nature as monoatomic gases because of their unreactive nature. The noble gas helium is used commercially in filling balloons because it has a density lower than that of air, causing the balloons to rise. Neon is commonly used in creating advertising signs because it glows when electricity flows through it. Argon is another noble gas and is used to keep light bulb filaments from burning away. Because argon is unreactive, it is inserted into a light bulb instead of regular air, which contains oxygen that can cause a burning reaction when exposed to the thin metal filaments found in light bulbs. Other noble gases include krypton, xenon and radon.
Some gases, called diatomic gases, consist of molecules made up of two atoms each. Examples of diatomic gases are hydrogen, carbon monoxide and oxygen, often abbreviated as H2, CO and O2. Other gases, such as steam, which is water in its gaseous state, and carbon dioxide, have three atoms per molecule. The chemical formulas for these gases are H2O and CO2.