What characterizes an isotope of a chemical element?

An isotope of a chemical element is characterized by having the same number of protons but a different number of neutrons in the nucleus. The number of protons in an atom defines the element itself and its position in the periodic table. By contrast, isotopes vary in their neutron count, which results in different mass numbers for those isotopes.

This variation in neutrons does not affect the chemical properties of the element significantly, as these properties are primarily determined by the arrangement and number of electrons surrounding the nucleus. Therefore, different isotopes of the same element tend to exhibit similar chemical behavior despite their differences in mass and nuclear stability.

The other options do not accurately describe isotopes: differing numbers of protons and electrons relate to ions, chemical properties are generally similar among isotopes, and the notion of "only one type of neutron" does not reflect the diversity of neutron arrangements present in isotopes.