Question

How can you justify 3rd period in periodic table contains 8 elements?

Answer 1

Simply put, the 3rd period has 3 electron shells and this outermost shell can support up to 8 electrons without touching the next shell even though 3d orbital exists.

This is partially because the the 4s orbital gets filled first, then the 3d since 3d orbitals have loser energy than 4p, but higher energy than 4s. So 4s gets filled first.