explain why there is a decrease in first ionisation energy between elements phosphorus and sulfur

The electrons are packed in singly occupied orbitals in the 3p subshell in phosphorus, however in sulfur, the 4th electron is placed into an orbital that already has an electron. Because electrons are negatively charged, they repel. This means it is easier to remove an electron and hence the first ionisation energy in sulfur is less than that in phosphorus