Question

What is a nuclide?

Answer 1

The nucleus of an atom

Explanation:

Chemical reactions happen because of the electrons of an atom - usually the valence electrons. The valence electrons form 2 Hydrogen atoms dance with the valence electrons from an Oxygen atom, and you've got Water...and a little flame.

Nuclear "reactions", though, don't usually involve the electrons (except electron capture). So an atom breaking apart (fission), fusion, alpha decay, beta decay, emission of gamma rays - all of these things in the atom only depend on the number of protons and neutrons. Too many neutrons or too few and the atom becomes unstable and radioactive.

Nuclide is basically referring only to the nucleus of the atom (with respect to nuclear processes), and is not concerned with the electrons or valence electrons, since they generally aren't involved in a nuclear process.