Answer:
Assuming the element Magnesium atom is neutral
Proton # = 12
Neutron # = 12
Electron # = 12
Step-by-step explanation:
In a neutral atom the number of protons and electrons is always equal
The atomic number tells the number of protons (12) in this case
The Atomic Mass describes the mass of the protons + mass of neutrons, so if you take 12 (number of protons) away you are left with 12 and some change(number of neutrons)