Question

It is said that the outermost shell of an atom can hold only 8 electrons. But if the shell is M shell, it should be able to hold 18 electrons(2n²). Expain me why it is nessasary for hte outer shell to have only 8 electrons when the shell can hold more than 8.

Answer 1

Ar has electronic config. as 1s2 2s2 2p6 3s2 3p6 here M shell has 8 electrons and hence Ar is stable.

The next noble gas Kr has electronic config 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 here the outer most shell is N shell not the M shell and has 18 electrons the N shell has 8 electrons but energetically electrons are added first to the 4s and 4p orbitals and then to the 3d orbital hence the outermost shell has 8 electrons.