<em>An element is the material that cannot be broken down further by chemical means.</em>
Explanation:
An element can be described as a material which is made up of only one kind of atom. For example, argon, hydrogen etc. An element cannot be broken down further by physical or chemical means.
Compounds, on the other hand, are made of different kinds of atoms. They can be broken down to simpler molecules by chemical means. For example, water is a compound made up of hydrogen and oxygen.
I think the layers of the atmosphere does temperature increase with increasing height. It is affected by convection because it heats the lower atmosphere. It is affected by conduction because the is the transfer of thermal energy. I guess
