Answer to Question #172069 in General Chemistry for Eon

Question

bcl3 is trigonal planar while ACl3 is tetrahedral in a dimeric state explain

Answer 1

BCl3 is trigonal planar atomic number of boron is 5, it's valence shell orbital is 3, In ground state one electon in s orbital two electrons in p orbital ,In

Excited one electon from s orbital goes into p orbital i.e. total 3 unpaired electrons one in s shell and two in p shell px and py this unpaired electrons get paired with one unpaired electron of chlorine atom so it form a sigma bond so it means total 3 sigma bond is formed one in s orbital and two in p orbital i.e. sp² hybridization is formed so it is trigonal plannar and bond angle is 120 degree

In AlCl3 molecule also form sp² hybridization but after hybridization in AlCl3 molecule the octet of Al is incomplete due to deficiency of two electrons in Al

To complete octet require 8 electrons but it has only 6 electron in Al atom of AlCl3 molecule

To complete it's octet its take lone pair of electrons from chlorine atom of another molecule of AlCl₃ and form Al₂Cl₆ molecule due to formation of co-ordinate bond so it form dimer in state and finally it forms sp³ hybridization so it forms tetrahedral in dimeric state

Boron atom not form in dimer due to small in size and short bond length repulsion occur.