ICSE Class 10 Answered

Moving across each period, you can see that the number of shells is the same as the period number.
As you go across each period from left to right the outer shell gradually becomes filled with electrons. The outer shell contains just one electron on the left hand side of the table, but is filled by the time you get to the right hand side.
Moving down each group, you can see that the number of electrons in the outermost shell is the same as the group number.
Each element in a group therefore has the same number of electrons in its outer shell.
Group 0 is a partial exception to this rule, since although it comes after Group 7 it is not called 'Group 8'; and it contains helium, which has only two electrons in its outer shell.

Cl < S< P < Si< Al < Mg < Na < Ar