# Sally is making a model of a Mg atom with an atomic mass number of 24. She has balls for the protons, neutrons, and electrons. She has added 6 neutrons to her model. How many more neutrons does she need to add to complete her neutral atom of magnesium?

For ""^24Mg.............................?
$Z$, $\text{the atomic number}$ of magnesium is 12. This means that there are 12 positively charged nuclear particles. This defines the particle as a magnesium atom.
To represent the ""^24Mg isotope, we thus need 6 more neutrons.