Hydrogen, carbon and oxygen commonly form covalent bonds. There are two forms of covalent bonds, polar and nonpolar, depending upon whether atoms share electrons equally.
Atoms are stable when their outer shell of electrons is full. If the shell is not full, atoms react with other atoms to gain, donate or share electrons. Covalent bonds are a sharing of electrons; an even sharing of electrons results in a nonpolar covalent bond. Nonpolar bonds mostly exist between atoms of the same element. For example, oxygen contains six electrons in its outer shell. To gain a full outer shell of eight electrons, oxygen atoms form double bonds with other oxygen atoms. Because they are identical atoms, they share the four electrons equally.
When atoms of two different elements share electrons, they form polar covalent bonds. In polar covalent bonds, the shared electrons spend more of their time near one atom than the other, resulting in an uneven sharing and a partial charge on both atoms in the bond. Water is an example of a polar covalent bond. In water, oxygen shares electrons with two hydrogen atoms. However, the oxygen has more pull on the shared electrons, giving it a partial negative charge and leaving each hydrogen with a partial positive charge.