Protons, neutrons and
Assuming the following element is neutral, it will have
electrons.
12
Mg
Magnesium
24.305
012, 12, 12
24, 24, 24
12, 24, 12
12, 12, 24

Assuming the element Magnesium atom is neutral

Proton #  = 12

Neutron # = 12

Electron # = 12

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)

it's c. elements are placed in rows based on how may valencies electrons have.