Which statement about isotopes is incorrect?

Isotopes are variations of the same chemical element that have the same number of protons but differ in the number of neutrons. This difference in neutron count leads to varying mass numbers for each isotope of an element, which is fundamental to understanding isotopes. The statement that isotopes "always have equal mass numbers" is false because, by definition, isotopes of an element will have different mass numbers due to the differing number of neutrons, even though they retain the same number of protons.

The other statements correctly highlight the characteristics of isotopes. They indeed have the same number of protons, which defines them as the same element, and they can vary in neutron count, leading to different isotopic forms. Thus, options that emphasize these characteristics provide an accurate picture of what constitutes isotopes.