Question

Which shells are called penultimate and prepenultimate shells?

Answer 1

Hint: We know that electrons are arranged in an atom in various shells around the nucleus and the Outermost shell of an atom is called valence shell and Pre means before.

Complete answer:
We know that shells are circular orbits in which electrons revolve around the nucleus. Firstly, we should know what the valence shell is. The valence shell is the outermost shell in which the last electron goes in an atom of an element. The shell inner to the valence shell is known as penultimate shell and the shell inner to the penultimate shell is called penultimate or anti-penultimate shell.

In the above diagram, we can see that N shell is the last shell i.e. valence shell.
So, we can clearly say that M is the penultimate shell and L is the prepenultimate shell.

Additional information:
-These shells i.e. penultimate and pre-penultimate shells are filled in case of representative elements and only valence shells are incomplete.
-In case of transition elements, the last two shells namely valence and penultimate shells are incomplete.
-In transition elements, the last shell contains one or two electrons. Penultimate shell may contain more than eight and up to eighteen electrons.
-In case of inner transition elements i.e. f block elements, the last three shells are namely valence, penultimate and penultimate shells are incomplete.
-In inner transition elements, the last shell contains two electrons and the penultimate shell may contain electrons more than eight or nine and pre-penultimate shell contains more than 18 up to 32 electrons.

Note:
-One must remember that the outermost shell of an atom is termed a valence shell.
-Secondly, it should also be remembered that the shell inner to valence shell is known as the penultimate shell.
-Last but not the least; one should also know that the shell inner to the penultimate shell is termed as the pre-penultimate shell.