Gases have no definite shape or volume, and take the shape and volume of their containers. In physics, a gas is a state of matter, consisting of a collection of particles (molecules, atoms, ions, electrons, etc.) without a definite shape or volume that are in more or less random motion.
Gaseous particles vibrate very rapidly, are relatively far far apart, and there are no forces holding them together.
Along with solids and liquids, gases make up he three states that matter can exist in.
 Characteristics of Gases
Gases have the following traits --
- Low density and viscosity compared to solids and liquids.
- Capable of expanding and contracting with changes in temperature or pressure./
- Diffuse readily when filled into a container.
 Greenhouse Gases
Greenhouse Gases have the ability to absorb and hold heat in the atmosphere, a phenomenon known as the "greenhouse effect." Many greenhouse gases occur naturally, such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Others such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) result exclusively from human industrial processes.