A gas always expands to fill a container. It is a physical state for which neither the shape or the volume are rigid. Gases can expand and contract over large temperature ranges. Gases are very compressible.

A gas can be a pure substance or a mixture. You know several elements which exist as gases and you know of at least one compound which can be found as a gas (water). Gases can also be mixtures. The interesting thing about mixtures of gases is that they are always homogeneous.

What does a gas look like at the atomic level?

The particles of a gas are well separated from each other and are in constant motion.