Question

An atom that has 13 protons and 15 neutrons is an isotope of the element? How can I count it?

Answer 1

it's simple.
the number of protons in an atom never changes. And also, the no of protons is its atomic number.if you look up the periodic table, you will find a number on the top - left corner of each of the boxes. This is the atomic number.No 2 same elements on the periodic table have the same atomic number. So, all you have to do is search for a box that has the number 13 on it.

and by the way ... the element which you're searching for is Aluminium