Describe the shapes of $BF_3$ and $BH_4^-$. Assign the hybridisation of boron in these species.

$(i)$ $\mathrm{BF}_{3}:$ As a result of its small size and high electro-negativity, boron tends to form monomeric covalent halides. These halides have a planar triangular geometry.

This triangular shape is formed by the overlap of three $s p^{2}$ hybridised orbitals of boron with the $s p$ orbitals of three halogen atoms. Boron is $s p^{2}$ hybridised in $\mathrm{BF}_{3}$.

$(ii)$ $\mathrm{BH}_{4}^{-}$: Boron-hydride ion $\left(\mathrm{BH}_{4}^{-}\right)$is formed by the $s p^{3}$ hybridisation of boron orbitals. Therefore, it is tetrahedral in structure.