Question

Does a $3rd$ electron shell have a capacity for $8{e^ - }$ or $18{e^ - }$ ?

Answer 1

The nucleus of an atom is surrounded by a number of shells. The electrons of the atom are located in these shells. One or more subshells normally make up an electron shell. The electrical configuration of the atom is determined by the electron shell.

Complete answer:
The first stable element in the $8A$ group of the periodic table is Helium which has electron configuration $1{s^2}$ .It has two electrons in its first shell.
The second stable element in the $8A$ group of the periodic table is Neon which has electron configuration $1{s^2}2{s^2}2{p^6}$ .It has two electrons in its first shell and $8$ electrons in its second shell.
The third stable element in the $8A$ group of the periodic table is Krypton which has electron configuration $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}$ .There are two electrons in first shell while the $2nd$ and $3rd$ shell contains $8$ electrons each.
From this we can say that the $3rd$ shell can hold $8$ electrons.
However in the $4th$ shell the $19th$ and $20th$ electrons go into the
$4{s^2}$ and not in the $3rd$ shell, but the next $10$ electrons go into the $3d$ orbitals that are a part of the $3rd$ shell but shown on the $4th$ shell level.
In this sense the $3rd$ shell can hold a total of $18$ electrons. Hence, the third shell can be considered to hold $8$ or $18$ electrons but in total the $3rd$ shell can hold $18$ electrons.

Note:
The number of electrons in an atom's outermost shell impacts its reactivity, or its tendency for forming chemical bonds with other atoms. The valence shell is the outermost shell, and the electrons in it are known as valence electrons.