An isotope is a variation of an element that has the same number of protons but a different number of neutrons. For example, carbon has three isotopes: carbon-12, carbon-13, and carbon-14. All three isotopes have six protons, but carbon-12 has six neutrons, carbon-13 has seven neutrons, and carbon-14 has eight neutrons.

The atom you described has 12 protons, so it is an isotope of magnesium. Magnesium has three stable isotopes: magnesium-24, magnesium-25, and magnesium-26. All three isotopes have 12 protons, but magnesium-24 has 12 neutrons, magnesium-25 has 13 neutrons, and magnesium-26 has 14 neutrons.

Therefore, the atom you described is an isotope of magnesium, specifically magnesium-24.