Given below are some elements of the modern periodic table:             ₄Be, ₉Fe, ₁₄Si, ₁₉K, ₂₀Ca

(i) Select the element that has one electron in the outermost shell and write its electronic configuration.

(ii) Select two elements that belong to the same group. Give reason for your answer.

(iii) Select two elements that belong to the same period. Which one of the two has bigger atomic size?

(i) ₁₉K has one electron in the outermost shell and its electronic configuration is 2 8 8 1. 

(ii) ₄Be and ₂₀Ca belongs to same group i.e., Group-2.

Electronic configuration of Ca and Be is:

₄Be - 2 2

₂₀Ca- 2 8 8 2

The number of electrons in the outermost shell of ₄Be and ₂₀Ca is same hence they belong to the same group.

(iii) ₉F and ₄Be belongs to the same period, Period 2.

Electronic configuration:

₉F - 2 7

₄Be - 2 2

₄Be has bigger atomic size then ₉F because the atomic radius decreases as we move from left to right. From left to right along a period, the nuclear charge increases and pulls the electrons closer to the nucleus. So, size of F decreases.