How do isotopes of a given element differ?

Isotopes of a given element differ in the number of neutrons they contain. While they have the same number of protons, which defines the element and determines its chemical properties, the variation in neutrons results in different nuclear masses.

For instance, carbon has isotopes like carbon-12 and carbon-14. Both isotopes have 6 protons, defining them as carbon, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. This difference in neutron count affects the atomic mass of the isotopes but does not change the chemical behavior of the element.

Thus, the correct answer highlights that isotopes are defined by their differing numbers of neutrons, which directly influences their stability and radioactivity, further elucidating the unique characteristics of each isotope.