Does the 3rd electron shell have a capacity for 8e- or 18e-?
The third shell in its lowest state has room for 8 electrons but including the higher energy 3d electrons it has a capacity of 18 electrons.
The first stable element in the VIII A group ( noble or inert gases) is Helium
The second stable element in the VIII A group is
The third stable element in the VIII A group is Krypton
In this sense the third shell can hold 8 electrons.
However in the fourth shell the 19th and 20th electrons go into the
In this sense the third shell can hold a total of 18 electrons.
So the third shell can be considered to hold 8 or 18 electrons but in total the third shell can hold 18 electrons.