How are sodium’s 11 electrons arranged in shells?

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The 11 electrons of a sodium atom are arranged in the first three electron shells as follows:

K shell (first shell): 2 electrons
L shell (second shell): 8 electrons
M shell (third shell): 1 electron

In the shell atomic model, this distribution is expressed as 2-8-1. The single electron in the outermost M shell is known as a valence electron, which accounts for the high reactivity and chemical properties characteristic of alkali metals.1