What distinguishes isotopes of the same element?

Isotopes of the same element are distinguished by having different numbers of neutrons in their atomic nuclei. While isotopes have the same number of protons, which defines the element and its chemical properties, the variation in the number of neutrons leads to different mass numbers for the isotopes. For example, carbon has several isotopes including carbon-12 and carbon-14, where both have six protons but differ in the number of neutrons (six and eight, respectively).

The differences in neutron count can influence the stability of the nucleus, leading to some isotopes being stable while others are radioactive. However, they retain the same chemical behavior because their electron configurations—determined by the number of protons—remain unchanged across isotopes. Thus, isotopes primarily differ in their physical properties, such as mass and stability, but not in their inherent chemical properties.