Isotopes are identified by the number of neutrons in the nucleus of an atom. While atoms of the same element have the same number of protons, which define the element itself, their neutron count can vary, leading to different isotopes of that element.
For instance, carbon-12 and carbon-14 are both isotopes of carbon. They have 6 protons, but carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons.
Thus, the subatomic particle that is crucial in determining the identity of isotopes is the neutron. This distinction is significant in fields like chemistry and nuclear physics, as different isotopes can have unique properties and behaviors.