Which periodic elements will most likely form an ion with a -3 charge?

Phosphorus (symbol P) when in a compound is an element that has a balance of 15 protons and 15 electrons; therefore it has no charge when in a compound. We know that it has 15 as it is number 15 on the periodic table.

When a phosphorus atom is ionized, it prefers to take on 3 electrons, giving it a new charge of -3, becoming phosphide. For example, if a chemist were to dissolve Na3P in water it will dissociate to form sodium ions and phosphide ions, leading to phosphorous having a charge of -3.