Guided Example
Determine the number of protons and neutrons in this isotope of magnesium: 24Mg
The mass number for magnesium is 24, as we know from the left-hand side superscript 24Mg.
If we look at the periodic table, we see that magnesium's atomic number is 12, so the number of protons = 12
If we subtract the atomic number from mass number, this will be the number of neutrons. So, 24 - 12 = 12