Isotope

Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

While the number of protons and electrons remain constant in the neutral atom, the number of neutrons may vary within different atom species of the same element. As a result, the atomic mass for one atom may be different from another atom of the same element if the number of neutrons varies.Atomic mass must account for all possible species or nuclides (isotopes), of an atom. Carbon 12 with its 6 neutrons is by far the most common isotope of carbon. In reality, there is a carbon 14 which has eight neutrons and an atomic mass of 14. There is also a carbon 11 which has only five neutrons.