The Covalent Bonds Form

An atom whose outer shell isn’t filled is chemically reactive, meaning that it can participate in chemical reaction with other atoms. An atom with an unfilled outer shell can share electrons with another atom. By sharing electrons, the atom can achieve a full outer shell and it will become more stable as a result. This situation is depicted in the figure below for hydrogen and oxygen, which can combine to form a water molecule. 

Hydrogen atom has just one electron in the outer shell instead of the allowed two, while oxygen atom has six electrons in the outer shell instead of the allowed eight. When two hydrogen atoms share their electrons with an oxygen atom, each atom now has a complete outer shell and is therefore more stable. Sharing of electrons between atoms is the basis of covalent bonds, whereby atoms are joined to one another to form molecules. The sharing of one pair of electrons gives rise to a single bond between two atoms and sharing of two pairs of electrons constitutes a double bond. The covalent bonds are typically very stable, which means it takes a substantial amount of energy to break them. The precise amount of energy is needed to break a covalent bond depends on the types of atoms involved and the number of bonds they form with one another. The double bonds generally being stronger than single bonds.

 

The above figure shows that covalent bonds in (A) a water molecule forms when each of two hydrogen atoms share an electron with an oxygen atom. Each hydrogen atom forms a single covalent bond with the oxygen atom. For figure (B) oxygen is commonly found as the molecule O₂, in which two oxygen atoms share electrons. Each shared electron pair represents a single covalet bond. The two atom oxygen atoms therefore form a double covalent bond with one another (Craig, dkk: 2010).