Question

How many elements are there in period 4?

Answer 1

The period in the periodic table:

1. The elements in the horizontal rows of a periodic table represent periods.
2. The 4th-period elements are the elements in the 4th row of the periodic table.

The number of elements in a period can be determined as:

Let 'p' be the period number, and

1. If p is even, the number of elements is: $\raisebox{1ex}{${\left(p+2\right)}^2$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$
2. If p is odd, the number of elements is: $\raisebox{1ex}{${\left(p+1\right)}^2$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$

Since period number is 4, it is even.

Therefore, number of elements in period 4 will be

$\frac{{\left(4+2\right)}^2}{2}=\frac{36}{2}=18$

Hence, there are 18 elements in period 4 of the periodic table.