Question

Which arrangement of the following set of atoms is in order of
increasing atomic radius: Na, Rb, K and Mg ;

(a) Na, Mg, K, Rb
(b) Na, K, Mg, Rb
(c) Mg, Na, K, Rb
(d) Na, Mg, Rb, K

Answer 1

Atoms is in order of increasing atomic radius : Na, K, Mg, Rb 

Answer 2

as we move down in a group the atomic radii increases so Na<K<Rb are in increasing order of atomic radii but if we move left to right in a group atomic radii decreases so Mg will have the smallest atomic radii as compared to other 3 elements as it belongs to group 2 so the correct answer will be (c) Mg, Na, K, Rb