For #Al#, its atomic number is 13 and its mass number is 27. How many neutrons does it have?
It necessarily has
The mass of a given atom depends on (i) the number of nuclear protons (and this number determines the identity of the element, because it gives
The other fundamental particle, the electron, is along for the ride, and its rest mass is miniscule compared to that of the nuclear particles. Nevertheless, the number of electrons, and how these are lost and gained by reaction with other substances, determines all of chemical reactivity.
And so in 2 paragraphs, we have covered all of chemistry and physics. Nice going!