The subatomic particle that is not a part of the nucleus which stays bound to the atom is the electron and is to maintain relative stability.
This is defined as a sub atomic particle which is negatively charged and revolve around the nucleus. In an atom the number of electron is usually equal to the number of the proton which is positively charged and present in the nucleus.
The electron not being in the nucleus is a a result of it wanting to maintain stability because its presence will produce a wavelength which will break the nucleus apart.
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