Work Step by Step
When naming an isotope, the name of the element is always followed by the atomic mass number, which equals the number of protons plus the number of neutrons. Because the atomic number of magnesium is 12, all magnesium atoms have 12 protons. The problem asks for an isotope of magnesium with an equal number of protons and neutrons, so there must be 12 neutrons as well. Thus, adding up the number of protons and neutrons to get an atomic mass number of 24, it is clear that this isotope is called Magnesium-24.