Explain why the first ionisation energy of sulfur is different from that of phosphorus.

The first ionisation energy is defined as, the amount of energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to produce 1 mole of gaseous ions, with a single positive charge. (By determining the electron configuration of sulfur and phosphorous, we can determine the amount of energy required to remove 1 mole of electrons) E.g. the electron configuration for sulfur is 1s²2s²2p⁶3s²3p⁴, and for phosphorous it is 1s²2s²2p⁶3s²3p³. In sulfur, the 4 electrons in the 3p level, are all paired. While in phosphorus there are 2 paired electrons and 1 lone electron in the 3p level. Due to the electron repulsion of the paired electrons in sulfur, less energy is required to remove 1 mole of electrons from its sub-level, as there is less of an attraction of these electrons towards its nucleus. Therefore, the first ionisation energy for sulfur will be slightly lower than that of phosphorus, due to the paired electrons in its 3p sub-level.