Isotopes of an element differ in what specific characteristic?

Isotopes of an element are defined by their differing numbers of neutrons while possessing the same number of protons. The number of protons in an atom determines the element itself, which is why all isotopes of a particular element share the same atomic number. However, variations in the number of neutrons lead to different mass numbers for these isotopes. For example, carbon has several isotopes, with carbon-12 having 6 neutrons and carbon-14 having 8 neutrons. This distinction in neutron count is critical, as it affects the stability and radioactive properties of the isotopes, but does not affect the chemical properties to a significant extent since these are primarily determined by the number of protons and electrons.