Which ion is formed when a phosphorous atom gains three electrons?

When a phosphorus atom gains three electrons, it achieves a more negative charge due to the additional negative charges contributed by the electrons. The atomic number of phosphorus is 15, meaning it has 15 protons in its nucleus. A neutral phosphorus atom has 15 electrons, balancing the positive charges of the protons.

By gaining three more electrons, the total number of electrons becomes 18, which results in a net charge of -3 (since there are 15 protons and 18 electrons). This is consistent with the formation of the phosphate ion, specifically PO4, which carries a -3 charge when phosphorus completely forms four bonded atoms.

Consequently, the correct answer, PO4 -3, reflects this complete gain of three electrons and the resultant anionic state of the phosphate ion. Understanding this transformation is key in chemical bonding and ionic charge calculations, especially in the context of phosphorus chemistry.