What defines an isotope of an element?

An isotope of an element is defined by its having a different number of neutrons than the most common form of that element. The presence of different neutrons results in variations in the atomic mass of the isotopes, while the atomic number—determined by the number of protons—remains constant. Thus, isotopes are variants of the same chemical element that exhibit identical chemical behavior but may have different physical properties, such as stability and radiological characteristics.

Since the number of protons remains the same for isotopes, this guarantees that they belong to the same element. The choice that identifies the difference in the number of neutrons accurately encapsulates the essence of what makes isotopes distinct while maintaining their identity as the same element.