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What is the defining feature of isotopes concerning their atomic structure?

  1. The number of neutrons varies

  2. The number of protons varies

  3. The number of electrons varies

  4. The mass number is always the same

The correct answer is: The number of neutrons varies

Isotopes are defined by the variation in the number of neutrons present in the atomic nucleus of an element, which means that they have the same number of protons but different mass numbers. The mass number is calculated by adding the number of protons and neutrons together, so when the number of neutrons changes while the number of protons remains constant, this results in different isotopes of the same element. For instance, consider carbon, which typically has 6 protons. The presence of 6 neutrons gives you carbon-12, while the presence of 7 neutrons results in carbon-13, and 8 neutrons leads to carbon-14. Thus, the defining characteristic of isotopes is indeed the variation in the number of neutrons in the nucleus. The number of protons, which determines the identity of an element, remains unchanged for all isotopes of that element, distinguishing isotopes from other atomic variations.