Describe the shapes of $BF $$_{3}$ and $BH _{4}^{-}$. Assign the hybridisation of boron in these species.

As a result of its small size and high electronegativity, 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 $BF _{3}$. (figure)

$(ii)$ $B H _{4}^{-}$

Boron-hydride ion $(BH_4^-)$ is formed by the $s p^{3}$ hybridisation of boron orbitals. Therefore, it is tetrahedral in structure.(figure)