Covalent bond represents shaired pair of electrons between the bonding atoms. When energy is supplied, it may cleave either symmetrically(HOMOLYSIS) or unsymmetrically(HETEROLYSIS).

 

(i). Homolysis: Both atoms joined by the covalent bond get one electron each and thus result in formation of radicals.

eg.

 

Note that since one electron is being transferred to each atome, the cleavage is shown by an arrow with a half arrow-head.

 

(ii). Heterolysis: Both electrons are taken up by one of the atoms(which is normally more electronegative than the other). as a result, ions are formed.

 

In this case, complete arrow-head is used.

 

 

Just like bond fission, arrows are used to show bond formation and same rule applies(half for radicals and full arrowhead for ions)

 

eg.