Question

What is the structure of ${\mathrm{BF}}_4^{-}$ ?

Answer 1

Step 1: Hybridization:

• “Hybridization is defined as the process of intermixing of the orbitals of slightly different energies so as to redistribute their energies, resulting in the formation of a new set of orbitals of equivalent energies and shape.”

Step 2: Formula used for calculation:

• $\mathrm{Steric}\mathrm{number}=\frac{1}{2}\left[\mathrm{V}+\mathrm{M}-\mathrm{C}+\mathrm{A}\right]$
• Where V is the number of valence electrons on the central atom, M is the monovalent electron, C is the positive charge and A represents the negative charge.

Step 3: Calculation for Hybridisation of ${\mathrm{BF}}_4^{-}$:

• The total number of valence electrons on the central atom in ${\mathrm{BF}}_4^{-}$ is 3 and there are 4 monovalent atoms and 1 negative charge.
• $$\begin{gathered} \mathrm{Steric}\mathrm{number}=\frac{1}{2}\left[3+4+1\right]=\frac{1}{2}\times 8 \\ \mathrm{Steric}\mathrm{number}=4 \end{gathered}$$
• The steric number is 4, thus the hybridization of ${\mathrm{BF}}_4^{-}$ is ${\mathrm{sp}}^3$.

Step 4: Structure and shape of ${\mathrm{BF}}_4^{-}$:

• As per VSEPR theory, for ${\mathrm{sp}}^3$ hybridized central atoms the only possible molecular geometry is tetrahedral.
• The structure of ${\mathrm{BF}}_4^{-}$ is given below: