In order to understand the chemistry of natural gas, it is important to be familiar with some basic concepts of general chemistry. Here are some definitions you should know: Matter — anything that has mass and occupies space. Energy — the capacity to do work or transfer heat. Elements — substances that cannot be decomposed into simpler substances by chemical changes. There are approximately 112 known elements. Examples: carbon, oxygen, and nitrogen. Atom — the smallest unit in which an element can exist. Atoms are composed of electrons, protons, and neutrons. Compounds — pure substances consisting of two or more different elements in a fixed ratio. Examples: water and methane. Molecule — the smallest unit in which a compound can exist or the normal form in which an element exists. Example: One molecule of water consists of two hydrogen atoms and one oxygen atom. One molecule of nitrogen consists of two atoms of nitrogen. Mixture — combination of two or more pure substances in which each substance maintains its own composition and properties. Examples: natural gas, gasoline, and air