Gases can be pure , where each molecule is made of individual atoms , elemental , where each molecule is made of more than one of the same atom bound together , or compounds where molecules are made of many types of atoms together . An example of a pure gas is Neon , an example of an elemental gas is Hydrogen and an example of a compound gas could be Carbon Dioxide . A pure gas may be made up of individual atoms ( e.g. a noble gas or atomic gas like neon ) , elemental molecules made from one type of atom ( e.g. oxygen ) , or compound molecules made from a variety of atoms ( e.g. carbon dioxide ) .