Question

Diagonal relationship in periodic table is shown by the elements of ___ and _____ periods:
(A) I and II
(B) II and III
(C) III and IV
(D) I and III

Answer 1

Hint: A diagonal relationship between two elements exists when they belong to two different groups and periods yet show a huge similarity in their chemical and physical properties. Such elements are usually placed along a diagonal in the periodic table.

Complete Step By Step Answer:
The elements that are the first members of every group show anomalous behavior. Their physical and chemical properties are different from those present in the remaining elements of the group. Such elements exhibit high electronegativity, have very small atomic sizes and the inability to show high oxidation states due the limitation of absence of d-orbitals.
These first members belong to the II period. These elements differ in properties from their group members but show a similarity in properties with the elements placed in the next period of the adjacent group. It appears as if they are diagonally placed and therefore this relationship is called a diagonal relationship.
Lithium and magnesium show considerable similarities in their properties and are said to have a diagonal relationship. Their similar sizes, nearly equal electronegativity and polarizing power is responsible for the similarity in their observed properties.
\[ \Rightarrow \] Thus, the elements that show a diagonal relationship belong to the II and III periods.
Option (B) is correct.

Note:
The diagonal relationship is due to the similar sizes and electronegativity values. Electronegativity decreases on descending a group and increases left to right, atomic sizes increase on descending a group and decreases on going left to right in a period, these opposing factors neutralize diagonally and a similarity in properties is observed.