Question

Why is BF3 a planar molecule, while NH3 is not?

Answer 1

BF3 consists of three B-F bonds and since B is in group III, it has no unshared pair of electrons. It does not have an octet of electrons. Count the regions of high electron density. It is 3 and 3 gives a trigonal planar structure. NH3, on the other hand, has a complete octet; therefore, it has a unshared pair of electrons as well as the three pairs shared with the three H atoms. Thus NH3 has an unshared pair of electrons plus the three N-H bonds to make four regions of high electron density which makes the electronic structure tetrahedral.