An atom of Na has a total of 11 electrons. Its electronic configuration is 2, 8, 1. But, Na+ ion has one electron less than Na atom i.e. it has 10 electrons. Therefore, 2 electrons go to K-shell and 8 electrons go to L-electrons, thereby completely filling K and L shells.
Mathematically
atomic number of sodium (Na) = 11
No. of electrons in Na = 11
No. of electrons in Na+ = 11 – 1 = 10
Electronic configuration of Na+ → 2, 8
Thus, in Na+, K and L shells are completely filled.
