Answer:
The atomic number of an element represents the number of protons in the nucleus of an atom. For example, carbon has an atomic number of 6, which means it has six protons in its nucleus.
The mass number of an atom represents the total number of protons and neutrons in the nucleus. For example, carbon-12 has a mass number of 12 because it has six protons and six neutrons in its nucleus.
Since the atomic number is equal to the number of protons in an atom, it is not possible for an atom to have 12 protons (atomic number of 12) and a mass number of 10 because the number of protons plus the number of neutrons cannot be less than the atomic number.
Therefore, an atom with an atomic number of 12 must have at least 12 nucleons (protons and neutrons) in its nucleus. The only stable isotope of magnesium, which has an atomic number of 12, has a mass number of 24, meaning it has 12 protons and 12 neutrons in its nucleus.