 
       
      
      exists as monomer whereas 
 is dimerised through halogen bridging. Give reason. Explain the structure of the dimer of 
 also.
  has 6 electrons in its valence shell and is in trivalent state. It requires two additional electrons to complete the octet. 
 act as a Lewis acid and 
 can donate its electron. Therefore, 
 readily accepts the lone pair of an electron from 
 and forms 
  makes a stable molecule by forming a Dimer. Aluminium’s valence shell has 6 electrons and it requires 2 additional electrons for completing the octet. While chlorine has 3 lone pairs of electrons. Therefore, Aluminium takes up 1 lone pair and forms Dimer with chlorine.
 
