Final answer:
Magnesium has an atomic number of 12, indicating 12 protons. To find the neutron number, the atomic number is subtracted from the mass number of a specific isotope. For the most common isotope of magnesium, 24Mg, there are 12 neutrons.
Step-by-step explanation:
The number of neutrons in the nucleus of a magnesium atom can be determined by subtracting the atomic number from the mass number of a specific magnesium isotope. Magnesium has an atomic number of 12, which means every magnesium atom has 12 protons. To find the number of neutrons, we need the mass number, which represents the total of protons and neutrons together. Different isotopes of magnesium have different mass numbers because they have a different number of neutrons. For example, the most common isotope, 24Mg, which makes up about 78.70% of naturally occurring magnesium, has a mass number of 24. Subtracting the atomic number (12 protons) from the mass number (24) reveals that this isotope has 12 neutrons. Therefore, statement a, which says it has 12 protons, 12 electrons, and 12 neutrons, is correct for 24Mg.