Question

the third period a containes only the 8 element even through the electron capacity of third shell is 18

Answer 1

It is because of the arrangement of electrons in accordance with the order of energy levels in the shells.
The diagram for the arrangement of electrons in the order of the energy levels is as follows
Here the electrons enter into the 3s shell which can accommodate 2 electrons and then enters into the 3p shell which can accommodate 6 electrons and then it does not the 3d shell, instead it enters the 4s shell.

This is the reason.