What is penultimate shells and antipenultimate shells?

The electrons are arranged in an atom in the various shells around the nucleus. The last shell or the outer most shell from the nucleus is called the valence shell. The shell inner to this is called the penultimate shell and the one inner to penultimate shell is called the anti penultimate shell.

According to the maximum no. of electrons that can be accommodated in any shell is given by 2n^2, where n represents the no. of shell.

Shell  2n^2

K  2

L   8

M  18

N  32

O  50

The maximum number of electrons that can be filled in the valence shell is 8, that in the penultimate shell is 18 and the anti penultimate shell has maximum capacity of 32 electrons.