Noun
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principal quantum number (plural principal quantum numbers)
(physics) The quantum number (of an atomic orbital) that specifies the energy of an electron in an atom and, hence, which shell it occupies
The principal quantum number in hydrogen is related to the atom's total energy. Source: Internet
These are n, the principal quantum number, for the energy, l, or secondary quantum number, which correlates to the angular momentum, ml, for the orientation, and ms the spin. Source: Internet