What are isotopes?

Isotopes are defined as atoms of the same element that differ in the number of neutrons. This means that while they have the same atomic number (which indicates the number of protons and defines the element), they have different atomic masses due to the variation in neutron count. For example, carbon has several isotopes, including carbon-12 and carbon-14; both have six protons (making them carbon) but differ in their neutron numbers, with carbon-12 having six neutrons and carbon-14 having eight. This distinction allows isotopes to have different physical properties while retaining the same chemical behavior, as chemical properties are largely determined by the number of protons and electrons involved in bonding.

Other choices do not accurately reflect the definition of isotopes. For instance, atoms of the same element with different numbers of protons would represent different elements altogether, and the same element cannot have different chemical properties solely based on variations in their neutron count.