Question

Diagonal relationship is not shown by:
A. $\text{Li and Mg}$
B. $\text{Be and Al}$
C. $\text{B and Si}$
D. $\text{C and P}$

Answer 1

Hint: A diagonal relationship exists between some pairs of elements which are diagonally adjacent and they are present in the second and third periods of the periodic table. Factors are the polarizing power which depends on ionic radius being almost similar to the diagonally placed elements because of which such relationships occur.

Complete answer:
First, let us know the reason of diagonal relationship; on crossing a period of the periodic table, the size of the atom decreases and on descending a group the size of the atom increases and other Let us now see the similarities in properties of the elements as outcome of diagonal relationship. Let us study the examples one by one.
A.$\text{Li and Mg}$:

S. No	Type of properties	SIMILARITIES shown by Magnesium and Lithium
1	Reaction with water	React slowly with water and are less soluble. Hydroxides decompose on heating.
2	Reaction with nitrogen	Formation of nitrides takes place by direct combination of elements with nitrogen, $\text{M}{\text{g}}_3{\text{N}}_2\text{ and L}{\text{i}}_3\text{N}$.
3	Carbonate and hydrogen carbonates	Carbonates decompose in heating to oxides and$\text{C}{\text{O}}_2$. Solid hydrogen carbonates are not formed.
4	Hydrates	Chlorides are deliquescent and crystallise from aqueous solution as hydrates, $\text{LiCl and MgC}{\text{l}}_2$.

B.$\text{Be and Al}$:

S. No.	Type of properties	SIMILARITIES shown by Aluminium and Beryllium
1	Not easily attacked by acids	Due to the presence of an oxide film on the surface of the metals, they are not easily attacked by acids.
2	Reaction with alkali	Reacts with excess of alkali to give beryllate and aluminate ions, ${\left[\text{Be}{\left(\text{OH}\right)}_4\right]}^{2-}\text{and }{\left[\text{Al}{\left(\text{OH}\right)}_4\right]}^{-}$.
3	Lewis acids	The chlorides of both are soluble in organic solvents and are strong Lewis acids as they are electron deficient. Also used as Friedel craft catalysts.
4	Complex Formation	Both the ions have the tendency to form strong complexes like${\text{BeF}}_4^{2-}{\text{ and AlF}}_6^{3-}$.

C.$\text{B and Si}$:

S. No	Type of properties	SIMILARITIES shown by Boron and Silicon
1	Exhibit property of non-metals	Are high melting and boiling point.
2	Different solid forms	Exist in crystalline and amorphous forms.
3	Conduction of electrons	Non- conductors of electricity and semiconductors also.
4	Ability to release${\text{H}}^{+}$ion	Are very weak acids, ${\text{H}}_3\text{B}{\text{O}}_3\text{ and }{\text{H}}_4\text{Si}{\text{O}}_4$.
5	Hydrolysis of chlorides	The reaction of hydrolysis is $\begin{array}{rl}& \text{BC}{\text{l}}_3+3{\text{H}}_2\text{O}\to \text{B}{\left(\text{OH}\right)}_3+3\text{HCl}\\ & \text{SiC}{\text{l}}_4+3{\text{H}}_2\text{O}\to \text{Si}{\left(\text{OH}\right)}_4+4\text{HCl}\end{array}$

D. C and P: Shows no similar properties.
So, the correct answer is “Option D”.

Note: The elements may be present in different groups and periods, but still resembles each other, due to small size. So, the resemblance occurs by size similarity not by just being present in the same group. Like, Li shows similarities with Mg but shows distinction in some properties with its own group (I).