The Electronic configuration of Lithium Li with atomic no 3 is 2,1. The electronic configuration of Na and Ar and the number of electrons in the K,L,M shells are as follows:
Element
Electronic configuration
K
L
M
Na
2,8,1
2
8
1
Ar
2,8,8
2
8
8
Since the outermost shell can have only 18 electrons, 4th period to 7th period has 18 elements.