National League for Nursing (NLN) Science Practice Exam

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To be isotopes, two atoms of the same element must have a different number of?

neutrons.
protons.
electrons.
quarks.

The correct answer is: neutrons.

Isotopes are defined as variants of the same chemical element that have the same number of protons but differ in the number of neutrons. Since the number of protons defines the element, two atoms that belong to the same element will always have the same number of protons. The distinction between isotopes lies in the number of neutrons, which contributes to differences in atomic mass but does not change the element's identity. This difference in neutron number can result in isotopes that may exhibit different nuclear stability, leading to some isotopes being radioactive while others are stable. Thus, understanding that it is the neutrons that vary while protons remain constant is fundamental to the concept of isotopes in chemistry and nuclear physics.