Question

For `Al`, its atomic number is 13 and its mass number is 27. How many neutrons does it have?

Answer 1

It necessarily has `14` `"neutrons"`.

Explanation:

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 `Z` by definition); and (ii) the number of neutrons, massive particles of neutral charge. Neutrons and protons are the massive fundamental particles, which determine the mass of a given isotope. And thus for `""^27Al`, we know that there are `"14 neutrons"`, because if it is an aluminum nucleus it MUST have `"13 protons"`.

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!