Na+ is a positively charged sodium ion. Normally an atom of sodium has 11 electrons, so according to formula it has 2 electrons in K shell , 8 electrons in L shell and 1 electron in M shell. However Na+ has 10 electrons only, therefore it has 2 electrons in K shell and 8 in L shell. 2 and 8 are the full capacity of the K and L shell respectively. Hence it is said ' Na+ has completely filled K and L shells. '