A gas has no particular shape. It fills its container. The atoms or molecules of a gas do not touch as they do in a liquid or solid.
Several of the nonmetallic elements are gases at room temperature. At a high enough temperature, any element vaporizes and becomes a gas. Here are described significant gases not covered elsewhere.
To discuss hydrogen is to discuss the phlogiston theory of burning. This obsolete theory arose with alchemists in the late medieval period. Phlogiston was a substance found in inflammable items and which was consumed by fire. The fire would go out when all the phlogiston was removed or when the air in an enclosed space could no longer contain any more of it. Scientists held this theory from about 1670 to 1777.
During and before this period, various scientists did chemical experiments that emitted a highly inflammable gas. It was the Englishman Henry Cavendish who purified and described the gas in detail. As this gas burned vigorously, if not explosively, and was lighter than air (heat rises, after all), he assumed that he had isolated phlogiston itself. Cavendish is credited after the fact with having discovered an element. The product of its burning is water, and so it is called hydrogen (hydro- "water", and -gen "creation").
Nitrogen is the largest component of the atmosphere. It is required for life by animals and plants, and so is a component of fertilizer. Nitrogen is somewhat inert chemically. It is used as the atmosphere in some food containers and for some works of art to avoid oxidation. Nitrogen has many, many uses as a chemical.
The Scotsman Danial Rutherford experimented with air around 1772. He "saturated the air with phlogiston" by a mouse's breathing and by a candle, then removed the carbon dioxide. His interpretation of the remaining three-quarters volume is incorrect by modern theories, but Rutherford is considered the discoverer of nitrogen.
Oxygen is about one-quarter of the atmosphere, but that is a result of the existence of plant life. Oxygen is highly reactive and combines with many things. Any free oxygen in the air must be continually produced, and so is a sign of life. Burning and rusting are the combination of oxygen with something else, and it is the fact that corroding metal gains weight that put an end to phlogiston.
Actual discovery of oxygen as a substance is credited to both Joseph Priestley (English) and Carl Wilhelm Scheele (Swedish) in the 1770s. This gas was very easy to breathe and supported fire vigorously.
Fluorine and chlorine, the gaseous halogens, are discussed here.
Helium and the other noble gases are discussed here.