Question

How would you justify the presence of 18 elements in the $5^{th}$ period of the periodic table.

Answer 1

In 5th prd, there are 2 s block elements and 6 p block as in 2nd & 3rd period. But from 4th period there is an increase in elements of 10 due to the presence of d block. In 4th & 5th there are 10 d block elements alongwith 2 s and 6 p. So, here are 18 elements.