Question

In 3 perod contain only 8 element even throughthe elrctron capacity is 18

Answer 1

This is due to the Aufbau rule of
$n + l$
Where n is principle quantum no. and l is azimuthal quantum no.

The value of n+l of 3d orbital is ( 3 + 2 = 5)
Which is less than that of 4s orbital (n+l value of 4)

Hence 4s orbital fills before 3d

Thus 4th period starts before completion of 3rd shell

Hence the incomplete 3rd shell consisting of 3s and 3p orbitals contribute maximum of 8 electrons

Therefore 3rd period has 8 elements