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QUESTION

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 1s^2  2 electrons in the first shell 2 electrons total The second stable element in the VIII A group is Neon  1s^2 2s^2 2p^6 8 electrons in the 2nd shell 10 e^- total The third stable element in the VIII A group is Krypton 1s^2 2s^2 2p^6 3s^2 3p^6 8 electrons in the 3rd shell 18e^-

In this sense the third shell can hold 8 electrons.

However in the fourth shell the 19th and 20th electrons go into the 4s^2 not the third shell, but the next 10 electrons go into the 3d orbitals that are part of the third shell but shown on the fourth shell level.

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.