Isotopic

A chemical element is determined by the number of protons in the nucleus which is always identical with the number of the electrons in the atomic shell (otherwise, the element would not be an atom but an ion ). Usually, a nucleus holds the same number of neutrons and protons, but there are some exceptions. In some elements, particularly very heavy ones, there are atoms with different numbers of neutrons. The atoms of an element having a different number of neutrons (and accordingly a slightly different mass) are referred to as the isotopes of the element.