Question

Why shoul the penultimate shell have only 8 electrons in some cases?

Answer 1

Dear student!

According to the rule,

In rows 4 - 7, the penultimate shell (2nd outermost shell), goes from 8 to 18 electrons. This happens between groups 2 and 3, without changing the outermost shell. So, if the innermost shell is full with two 1s electrons then next-to-outermost shell is full with two 2s electrons and six 2p electrons, for a total of 8. So, no. of electrons in the penultimate shell depends on the no. of electrons in its outermost shell and innermost shell both.