Isotopes of an element differ in their number of neutrons.

Here's why:

* Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.

* Protons determine the element's identity (atomic number).

* Neutrons contribute to the atom's mass but don't affect its charge.

So, while all magnesium atoms have 12 protons, they can have varying numbers of neutrons, leading to different isotopes like magnesium-24, magnesium-25, and magnesium-26.