Symbolized by the letter n, this number describes the shell that an electron resides in. An electron “shell” is a region of space around an atom’s nucleus that electrons are allowed to exist, corresponding to the stable “standing wave” patterns of de Broglie and Bohr. Electrons may “leap” from shell to shell, but cannot exist between the shell regions.
The principal quantum number must be a positive integer (a whole number, greater than or equal to 1). To be precise, principle quantum number for an electron cannot be ½ or -3. These integer values were arrived at through experimental evidence of light spectra: the differing frequencies (colors) of light emitted by excited hydrogen atoms follow a sequence mathematically dependent on specific, integer values as illustrated below:
Each shell has the capacity to hold multiple electrons. The maximum number of electrons that any shell may hold is described by the equation 2n2, where “n” is the principal quantum number. Therefore, the first shell (n = 1) can hold 2 electrons; the second shell (n=2) 8 electrons, and the third shell (n=3) 18 electrons.
Also Read: Hadrons vs. Leptons
Previously, electrons in an atom were designated by letter rather than by number. The first shell (n=1) was labelled K, the second shell (n=2) L, the third shell (n=3) M, the fourth shell (n=4) N, the fifth shell (n=5) O, the sixth shell (n=6) P, and the seventh shell (n=7) Q.
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