Find out the total number of species which can dimerize.

${\mathrm{BeCl}}_2,{\mathrm{AlF}}_3,{\mathrm{AlCl}}_3,{\mathrm{BeH}}_2,{\mathrm{CaH}}_2,{\mathrm{ClO}}_3,{\mathrm{NO}}_2,{\mathrm{CO}}_2$.

The total number of species which can dimerize is ${\mathrm{BeCl}}_2,{\mathrm{AlCl}}_3,{\mathrm{BeH}}_2,{\mathrm{ClO}}_3,{\mathrm{NO}}_2$ 

${\mathrm{BeCl}}_2$ exists as dimer in the vapour state and polymeric in the solid state. Aluminium chloride occurs as a dimer when it is in a liquid or molten form.