Final answer:
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, causing a difference in mass numbers. For example, carbon has isotopes like carbon-12 with six neutrons and carbon-14 with eight neutrons, the latter being radioactive due to extra neutrons.
Step-by-step explanation:
Definition Of Isotopes
Isotopes are atoms of an element that have the same number of protons but a different number of neutrons in their nuclei, giving them different mass numbers. For instance, all carbon atoms have six protons, making up their atomic number. Yet, the number of neutrons can vary, as seen in carbon's three natural isotopes: carbon-12, carbon-13, and carbon-14. While carbon-12 and carbon-13 are stable, carbon-14 is a radioactive isotope due to its additional neutrons.
Understanding Mass Number
To determine the number of neutrons in an atom, one can subtract the atomic number (the number of protons) from the atom's mass number. The mass number (A) of an atom is the sum of the protons and neutrons in its nucleus. Hydrogen serves as a simple example, with its most common form having one proton and no neutrons (mass number 1), while its isotopes deuterium and tritium have one and two neutrons, respectively, giving them mass numbers of 2 and 3.