The atomic number of sodium is 11. It has 11 electrons in its orbitals wherein the number of protons is equal to the number of electrons. Hence, its electronic configuration is K-2 ; L-8 ; M-1 ; The one electron in the M shell is lost and it obtains a positive charge, Na+ .
The new electronic configuration is K-1 ; L-8 in which both K-shell and L-shell has completely filled electrons. Hence it is very difficult to remove the electrons from a filled state/inert state as it is very stable.