How can isotopes of a given element differ?

Isotopes of a given element differ primarily by their mass number. Mass number is defined as the total number of protons and neutrons in the nucleus of an atom. While isotopes have the same atomic number (and thus the same number of protons, which defines the element), they can have different numbers of neutrons. This variation in neutron count results in different mass numbers for the isotopes of that element.

Since the chemical properties of an element are mostly determined by the arrangement of its electrons, and electron arrangement is influenced by the number of protons, all isotopes of an element typically exhibit very similar chemical behavior. Isotopes also do not differ in charge because they are electrically neutral, with protons equaling electrons in an uncharged atom.

In summary, isotopes are categorized by their mass number, which reflects the different numbers of neutrons within the nuclei of the isotopes for a particular element.