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

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