American Chemical Society (ACS) Chemistry Practice Exam 2025 - Free ACS Chemistry Practice Questions and Study Guide

Isotopes are defined as forms of the same element that differ in the number of neutrons. This distinction is crucial because while isotopes have the same number of protons and thus belong to the same element, the varying number of neutrons leads to differences in their mass numbers. For example, carbon-12 and carbon-14 are both isotopes of carbon; they have 6 protons (which define them as carbon) but differ in neutrons (12 has 6 neutrons, while 14 has 8 neutrons).

Understanding this concept allows for a better grasp of various applications in chemistry and other sciences, such as radiocarbon dating and nuclear medicine, where specific isotopes are utilized based on their unique properties resulting from differing neutron counts. This differentiation also explains how some isotopes can be stable while others may be radioactive, highlighting the importance of neutron numbers in nuclear stability.

When considering other options, it becomes clear why they do not fit the definition of isotopes. The incorrect options suggest alternative definitions that either misidentify atomic structures or confuse fundamental concepts of atomic properties.