There are some atoms that are in a stable state when they are alone because their charge is balanced. Helium is one example of this type of atom. However, other atoms have an unbalanced charge, which causes them to want to share electrons with other atoms in order to balance their charge. When they share electrons, they form bonds in which the two atoms stick together to form a molecule. This bonding helps stabilize the atoms.
Some atoms may have a tendency to stack on one another in patterns, which forms crystals. These atoms, like other atoms that combine, seek to find a stabilized state, but instead of stopping once they've bonded with another atom, they continue to stack on one another. Fusion is another way atoms combine, and it occurs when the nucleus of two atoms forcefully combine to form one large atom, instead of forming a molecule with two atoms that share electrons.
Atoms that share one pair of electrons with another atom participate in ionic, metallic or Van der Waals bonds. Scientists refer to sharing electrons in pairs because each atom lends an electron to another atom, resulting in two total atoms that are shared. Bonds are called ionic when they occur between one metal atom and another nonmetal atom. Metallic bonds occur between two metal atoms. Van der Waals bonds consist of long strands of carbon that bond to other unstable atoms.
The process by which atoms share multiple pairs of electrons is called "covalent bonding." These bonds may be called double or triple bonds. Double bonds occur when atoms share two pairs of electrons, and triple bonds occur when atoms share three pairs of electrons. For example, the combination of sodium and carbon to form sodium bicarbonate is a double bond. In some rare cases, atoms may share more than three pairs of electrons.