<P> The number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons in the neutral (non-ionized) atom . Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons . The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number . </P> <P> For example, carbon - 12, carbon - 13 and carbon - 14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively . The atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that the neutron numbers of these isotopes are 6, 7 and 8 respectively . </P> <P> A nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus, for example carbon - 13 with 6 protons and 7 neutrons . The nuclide concept (referring to individual nuclear species) emphasizes nuclear properties over chemical properties, whereas the isotope concept (grouping all atoms of each element) emphasizes chemical over nuclear . The neutron number has large effects on nuclear properties, but its effect on chemical properties is negligible for most elements . Even in the case of the lightest elements where the ratio of neutron number to atomic number varies the most between isotopes it usually has only a small effect, although it does matter in some circumstances (for hydrogen, the lightest element, the isotope effect is large enough to strongly affect biology). The term isotopes (originally also isotopic elements, now sometimes isotopic nuclides) is intended to imply comparison (like synonyms or isomers), for example: the nuclides,, are isotopes (nuclides with the same atomic number but different mass numbers), but Ar,, Ca are isobars (nuclides with the same mass number). However, because isotope is the older term, it is better known than nuclide, and is still sometimes used in contexts where nuclide might be more appropriate, such as nuclear technology and nuclear medicine . </P> <P> Isotopes are different kinds of atoms of the same element which have the same numbers of protons (atomic number), but the different numbers of neutrons and different mass number (total number of protons and neutrons). </P>

A term that is used to describe (only) different nuclear forms of the same element is