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Explain valence shell and penultimate shell.

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Hint: Valence shell is the outermost shell in which we are filling the electrons and the penultimate shell is the one which is filled just before valence shell.
In case of sodium 3s is the valence shell while 2p is the penultimate shell.

Complete answer:
In an atom, the electrons are arranged in the various shells around the nucleus. Each shell has specific capacity up to which it can hold electrons. After one shell is filled, the electrons are entered in 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.
Example :- Sodium metal has atomic number 11.
Its electronic configuration can be written as :- $1{s^2}2{s^2}2{p^6}3{s^1}$
So, here 3s is the valence shell of sodium atom and the one electron in its 3s is the valence electron.
The electrons in valence shells determine the way in which an atom will combine with other atoms. Valency is measured from the number of valence electrons.
Now, let us move to the second definition which is the penultimate shell.
Penultimate shell is the shell which is filled just before the valence shell i.e. the shell just inner to valence shell. It will have a number one less than valence shell. If the valence shell has value ‘n’ then the penultimate shell will have value (n-1).
Example :- The Magnesium has atomic number 12. It is a block element.
Its electronic configuration can be written as :- $1{s^2}2{s^2}2{p^6}3{s^2}$
So, the 3s is the valence shell here and 2p is the penultimate shell.

Note: The valence shell electrons determine the chemical properties of an element. This decides about the bonding of the atoms and its 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.