Question

Bf3 exist but bh3 does not exist ncert answer

Answer 1

Bh3 molecule is electron deficient in nature since 'B' atom has only 6 electrons around it. So it exists in the form of dimer that is B2H6.
Bf3 is also deficient but because of larger size of 'F' atom, it can't undergo dimarization.

Answer 2

Explanation:

Bh3 molecule is an electron deficient molecule in nature since the 6 electrons are around the 'B' atom. Hydrogen being small in size, plus electron deficient it cannot donate electrons to boron, hence to exist, Bh3 goes under dimerization so as to fulfill the electron deficiency and attain stability.

Hydrogen being small in size dimerization is possible.

Bf3 is also deficient but because of larger size of 'F' atom, it cannot undergo dimerization, so it exists as Bf3 with a partial negative charge in boron and partial positive charge on fluorine.