Question

Give the number of neutrons in Al+3

Answer 1

The charge on Al doesn't matter for determining number of neutrons so disregard it.

An atom is defined by how many protons it has, Al has 13, so that's why it has that number on the periodic table. Now if you look there's another number, 26.98. This is the total number of nucleons (protons and neutrons) that an atom has on average. That's because the number of neutrons an atom has is not a constant, it can gain or lose them, so this is what you would normally find. This means that you can expect aluminum atoms have 27 nucleons, but sometimes more or less if you are going to round the number off.

So 27 is the number of neutrons and protons, and aluminum has 13 protons, so you subtract to find the neutrons, that's really all it comes down to. Atoms with different numbers of neutrons are called isotopes, and they're pretty important too.