Final answer:
The total number of electrons for the Lewis structure of the PO4 3− ion is 32, which includes the valence electrons of phosphorus and oxygen, plus additional electrons due to the ion's charge.
Step-by-step explanation:
The total number of electrons to be counted for the Lewis structure of the PO43− polyatomic ion is calculated by combining the valence electrons of phosphorus with those of oxygen, and then adjusting for the ion's charge. Phosphorus has five valence electrons, while each oxygen atom has six. Since there are four oxygen atoms, this amounts to a total of 24 valence electrons from oxygen. Adding the five valence electrons from phosphorus gives 29, and then we account for the −3 charge of the ion by adding three more electrons, resulting in a total of 32 valence electrons for the PO43− ion. These electrons are to be distributed to complete the octets around oxygen atoms and to satisfy the bonding requirements of phosphorus as the central atom.