Question

What causes isotopes?

Answer 1

Isotopes are atoms of the same element which have a different number of neutrons.

Explanation:

There are three isotopes of hydrogen. All have one proton - if they had two protons they'd be a helium atom!

The three isotopes are:
H-1
H-2
H-3

The number which follows the symbol is the mass number of each isotope (version) of hydrogen. For example; the mass of H-1 is 1 atomic mass unit (amu), the mass of H-3 is 3 atomic mass units.

The difference in mass is the result of the differing numbers of neutrons in the isotopes.

H-1 has 1 proton and 0 neutrons = total mass is 1
H-2 has 1 proton and 1 neutron = total mass is 2
H-3 has 1 proton and 2 neutrons = total mass is 3

Protons and neutrons have a mass which is extremely close to 1 amu, it's so close that you can typically just round to 1. Electrons have virtually no mass - so I haven't discussed them in this answer.

Here's a video to help with this concept.

Hope this helps!