Question

In ${\mathrm{PO}}_4^{3-}$, the formal charge on each oxygen atom and the $\mathrm{P}-\mathrm{O}$ bond order respectively are

• A. $-0.75,0.6$
• B. $-0.75,1.0$
• C. $-0.75,1.25$
• D. $-3,1.25$

Answer 1

The correct option is C

$-0.75,1.25$

The explanation for the correct answer:

Option (C) $-0.75,1.25$

Step 1 Bond order of ${\mathrm{PO}}_4^{3-}$

$\mathrm{Bond}\mathrm{order}=\frac{\mathrm{Number}\mathrm{of}\mathrm{bonds}}{\mathrm{Number}\mathrm{of}\mathrm{resonating}\mathrm{structures}}$

The resonating structures of phosphate ions are,

The total number of bonds is 5

The total number of resonating structures is 4

Hence, the Bond order is$\frac{5}{4}=1.25$

Step 2 Formula charge of ${\mathrm{PO}}_4^{3-}$

$\mathrm{Formula}\mathrm{Charge}=\mathrm{Valence}\mathrm{electrons}-\mathrm{Number}\mathrm{of}\mathrm{non}\mathrm{bonding}\mathrm{electrons}-\frac{\mathrm{No}.\mathrm{of}\mathrm{bonding}\mathrm{electrons}}{2}$

For oxygen atom that forms a double bond with Phosphorus atom,

The valence electrons are 6

The number of the non-bonding electrons is 4

The number of bonding electrons is 4

$$\begin{gathered} \mathrm{Formula}\mathrm{charge}=6-4+\frac{4}{2} \\ =6-4+2=0 \end{gathered}$$

For oxygen atom that forms a single bond with Phosphorus atom,

The valence electrons are 6

The number of the non-bonding electrons is 6

The number of bonding electrons is 2

$$\begin{gathered} \mathrm{Formula}\mathrm{charge}=6-6+\frac{2}{2} \\ =6-6+1=-1 \end{gathered}$$

In the resonance structure, a total of $-3$ charge is distributed over four oxygen atoms.

Thus, the formal charge of each oxygen atom is $-\frac{3}{4}=-0.75$
Therefore, in ${\mathrm{PO}}_4^{3-}$, the formal charge on each oxygen atom is $-0.75$ and the $\mathrm{P}-\mathrm{O}$ bond order is $1.25$.

Hence, option (C) is the correct answer.