Atoms of the same element with different numbers of what are called isotopes?

Atoms of the same element that differ in the number of neutrons are referred to as isotopes. Isotopes maintain the same number of protons, which defines the element and its chemical behavior, yet have varying numbers of neutrons. This variation in neutrons results in different atomic masses while still retaining the same chemical properties.

For example, carbon has isotopes like carbon-12 and carbon-14, where both have six protons (defining them as carbon), but carbon-12 has six neutrons while carbon-14 has eight. This characteristic of isotopes is significant in various fields, including radiocarbon dating, where carbon-14 is used to determine the age of ancient organic materials.

Other terms in the question, such as electrons and protons, refer to different subatomic particles that impact an atom's charge and identity, but do not define isotopes. Quarks make up protons and neutrons but aren't directly relevant to the definition of isotopes itself. Thus, the correct identification of isotopes hinges on the variation in neutron count.