Question

What is the formal charge on oxygen in $P - O$ bond of $PO_4^{ - 3}$ ?

Answer 1

Hint: Formal charge of an atom in a molecule is calculated by the total charge present on a molecule divided by the total atoms present in that molecule of which formal charge is to be calculated. One needs to find out the charge on the molecule and the number of atoms in that molecule to find the answer.

Complete step by step answer:
1) In the question, the formal charge on the oxygen atom has been asked in the molecule $PO_4^{ - 3}$ First, we will see the structure of the molecule $PO_4^{ - 3}$ which is shown below,

2) The molecule $PO_4^{ - 3}$ shows the resonating structure with each atom of oxygen at a time and shows the same structure as shown above. That means all the four oxygen atoms are similar to each other i.e. equivalent to each other and they possess the same charge on them irrespective of their position in the structure.
3) To calculate the formal charge on the oxygen atom we need to use the following formula,
${\text{Formal charge on oxygen = }}\dfrac{{{\text{Total charge on a molecule}}}}{{{\text{Total number of oxygen atoms}}}}$
4) The total charge on a molecule $PO_4^{ - 3}$ is $ - 3$ which can be taken as $3$ we need to ignore the negative or positive value. The total number of oxygen atoms in the structure is four which is the denominator value.
5) By putting the value in the formal charge formula we get,
${\text{Formal charge on oxygen = }}\dfrac{3}{4} = 0.75$
6) Therefore, we have calculated the formal charge present on oxygen in the molecule $PO_4^{ - 3}$ is $0.75$
Note:
There is an alternative method to calculate the formal charge which can be calculated by the number of valence electrons of an atom (which is the group number of that atom in the periodic table) minus the number of covalent bonds it has attached minus the number of lone pair of electrons on that atom (if any present). In the given molecule, the structure has resonance in it hence all the four oxygen atoms are similar to each other which can be said as equivalent.