Question

Which pair is different form the others
(A) Li – Mg
(B) B – Al
(C) Na – K
(D) Ca – Mg.

Answer 1

(A) Li – Mg – Lithium is alkali metal having atomic number $3$ belongs to group I magnesium is alkaline earth metal having atomic number $12$ belongs to Group – II.
(B) B – Al: Boron is a metalloid having atomic number $5$, belong to Group – $13$ aluminium is a metal having atomic number $13$, belongs to group – 13
(C)Na – K
Sodium (Na) is an alkali metal having atomic number $11$, belongs to the group – I potassium (K) is also an alkali metal having atomic number $1g$.
(D) Ca – Mg
Calcium (Ca) is an alkaline earth metal having atomic number $20$ and magnesium (Mg) is also alkaline earth metal having atomic number $12$.

Complete step by step answer:
(A) Li – Mg – lithium and magnesium both belong to the s – block because the last electron enters the outermost S – orbitals but lithium belongs to group – I and magnesium belongs to group – II Li – Mg has a diagonal relationship. A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the ${2^{nd}}$ and ${3^{nd}}$ period. Diagonal relationship explain about the charge density $L{i^ + }$ is a small carbon with $+ 1$ charge and $M{g^ + }$ is somewhat larger with $+ 2$ charge
(B) B – Al – Boron and aluminium both belong to p – block elements as the last electron enters the outermost p – orbitals also both belong to the same group I.e. groups
(C) Na – K – Sodium and potassium both are belongs to s – block element as last electron enters the outermost s – orbital and also belongs to same group i.e. group – I
(D) Ca – mg – Calcium magnesium both belong to s – block element and also from the same group – II.
Hence, the correct option is (A).

Note:
Diagonal relationships occur due to crossing and descending the periodic table have opposite effects. The diagonal relationship is due to the similarity in ionic sizes or charge or radius ratio of the elements.