How many protons, neutrons, and electrons does a magnesium atom with a mass number of 25 contain?

Nov 12, 2015

$Z$, the atomic number of $M g$ is $12$.

Explanation:

$Z$ refers to the number of (nuclear) protons. The $M g$ nucleus contains 12 positively charged particles (how can I make this statement?). To balance this charge, (the metal is electrically neutral after all), there must be 12 negatively charged particles associated with the protons; these are called electrons and they are conceived to orbit around the positively charged nucleus.

But the negatively charged particles have negligible mass. How do we account for the mass number of 25? Well, there must be 13 particles of zero charge, neutrons, and these reside in the nucleus.