COVALENT BONDING
Atoms are most stable when all of their orbitals are occupied by two electrons. Isolated atoms, such as hydrogen (H), carbon (C), nitrogen (N) and oxygen (O), have some orbitals that are not occupied by two electrons. For example, hydrogen is an atom that has a single proton (its nucleus) and a single electron. The electron of the hydrogen atom is in a 1s orbital.

Two isolated hydrogen atoms can come together and merge their 1s orbitals, each containing one electron, into a new, combined "bonding orbital" with two electrons. Their bonding orbital holding two equally shared electrons is a covalent bond.

The hydrogen atom can also form bonding orbitals with many other kinds of atoms that have an orbital containing a single electron. Since bonding orbitals contain two electrons (one from each atom), bonding orbitals are more stable than the orbitals of isolated atoms, and the resulting molecules are more stable than the isolated atoms.