Which shells are called penultimate and pre-penultimate shells?

Hint: The valence shell is the outermost shell in which electrons are being filled. Penultimate and pre-ultimate shells lie just before the valence shell.

Complete step by step answer:
- In an atom, the electrons are arranged in the various shells around the nucleus. Each shell has a specific capacity up to which it can hold electrons. After one shell is filled, the electrons are entered into the next shell on the basis of the energy of these shells.
- Valence shell is the outermost shell of an atom that contains electrons. It may be fully filled or half-filled or somewhat in between. The electrons in these valence shells are called valence electrons.
- Penultimate shell is the shell that is filled just before the valence shell i.e., the shell just inner to valence shell.
- If the valence shell has value ‘n’, then the penultimate shell will have value (n-1).
- Pre-penultimate is the shell inner to the penultimate shell. This shell is filled just before the penultimate shell. It is also called anti-penultimate shells.

Note: The valence shell electrons determine the chemical properties of an element. This decides about the bonding of the atoms and their various other properties. If the valence shell is fully filled then it is most stable and thus, it does not react. Normally penultimate shells are considered filled but transition metals contain unpaired electrons in the penultimate shell.