24, because to get the AMU of an element you just add the number of protons and neutrons, because of the fact that electrons weight next to nothing.
Answer: The mass number of magnesium atom is 24 amu.
Explanation:
Atomic number is defined as the number of protons or electrons that are present in a neutral atom.
Atomic number = number of protons = number of electrons
Mass number is defined as the sum of number of protons and neutrons that are present in an atom. It is expressed in 'amu'
Mass number = Number of protons + Number of neutrons
For the given element, "magnesium"
Number of protons = 12
Number of neutrons = 12
Number of electrons = 12
Mass number = 12 + 12 = 24
Hence, the mass number of magnesium atom is 24 amu.
