The atomic number of phosphorus (P) is 15, which means it has 15 protons. In a neutral atom, the number of electrons is equal to the number of protons; therefore, a neutral phosphorus atom also has 15 electrons.
In a P³⁻ ion, phosphorus has gained 3 extra electrons. So, to find the number of electrons in the P³⁻ ion, you would add 3 to the 15 electrons:
15 electrons + 3 electrons = 18 electrons.
Thus, a P³⁻ ion has:
- 15 protons (which remains unchanged)
- 18 electrons (due to gaining 3 additional electrons)
The correct answer is: 15 protons and 18 electrons.