Definition of a MoleculeMolecules are formed when two or more atoms are held together by chemical bonds.
Molecules Formed by Covalent BondsFor example, the oxygen molecule consists of two atoms of oxygen. In the case of oxygen, the bond that holds the two atoms together is known as a covalent bond, and here is how it works.
The oxygen atom has 8 each of protons, neutrons, and electrons. The protons and neutrons are found in the center of the atom, known as the nucleus, and the electrons surround the nucleus in layers, or shells. The oxygen atom has 2 electrons in its first shell, and 6 in its second and outer shell. However, in chemistry, there exists the octet rule, which states that atoms generally strive to have 8 electrons in their outer shell. The oxygen therefore is 2 electrons away from a complete outer shell. When it binds with another oxygen atom, they can share two pairs of electrons and so each will have 8. When atoms share electrons like that, they form a molecule through a covalent bond.