Introduction
All matter is composed of elements. Examples of elements are hydrogen, oxygen and carbon. Each element is composed of atoms of a unique type. Atoms are composed of subatomic particles including protons, electrons and neutrons. These particles have positive, negative and neutral (no) charge, respectively. The protons and neutrons are in the nucleus of the atom and the electrons spin around the nucleus in paths called orbitals. Sets of electrons occupy concentric shells around the nucleus, like planetary orbits around the sun. Atoms are electrically neutral, having the same number of electrons as protons, except when they exist as ions.