Final answer:
The isotope ³¹P³⁻ has 15 protons, 16 neutrons, and 18 electrons. The number of electrons has increased by 3 compared to the neutral state due to the 3- charge.
Step-by-step explanation:
The student is asking about the number of protons, neutrons, and electrons in a phosphorus isotope with a 3- charge, specifically ³¹P³⁻. Phosphorus (³¹P) has an atomic number of 15, which means it has 15 protons. Since the mass number (indicated by the superscript) is 31, the number of neutrons is the mass number minus the atomic number, which is 31 - 15 = 16 neutrons. This specific isotope has gained 3 extra electrons, giving it a 3- charge. Therefore, in this ionized form, it contains 15 protons, 16 neutrons, and 18 electrons.