Dr Inam Ul Haq Jazbi Unit # 1 … Periodic Table (Diagonal Relationship)
Diagonal Relationship
Definition of Diagonal Relationship
The element of the 2nd period belonging to the IA, IIA and IIIA group is similar in behaviour to the element of
IIA, IIIA and IVA group respectively in the 3rd period. This Across-the-street similarity in properties between
two elements is called diagonal relationship.
The similarity in properties observed between two elements placed diagonally from
left to right in two adjacent periods and two adjacent groups in the periodic table is
known as the diagonal relationship.
OR
The close resemblance (similarity) of the properties between the first members of
second period with their diagonally opposite element of 3rd period belonging to next
group is called diagonal relationship.
The two elements showing similarities in properties are called Diagonal Neighbours or Diagonal Pair or
Bridge Elements. The diagonal members have similar atomic radii and polarising power i.e.
charge/size ratio is same.
Diagonal relationship is prominent between following pairs:
Reason of Diagonal Relationship
The diagonal relationship is attributed because of following reason:
1. Same charge density or Polarising Power or Ionic Potential (charge/size ratio)
2. Same Electronegativity
3. Same Reduction Potential
Conceptual Inorganic Chemistry Text Book -1-
, Dr Inam Ul Haq Jazbi Unit # 1 … Periodic Table (Diagonal Relationship)
1. Same charge density or Polarising Power or Ionic Potential
The diagonal pairs have almost same charge density (which is equal to charge/radius ratio) due to their
same ionic sizes. e.g.
Charge density of Be2+ = 2/0.31 = 6.45
Charge density of Al3+ = 3/0.5 = 6.0
2. Same Electronegativity
The diagonal pairs have almost same electronegativity. e.g. electronegativity of both Be and Al are same
(i.e. 1.5). Similarly electronegativity of Li is 1.0 and that of Mg is 1.2 and electronegativity of B is 2 and that
of Si is 1.8.
3. Same Reduction Potential
The diagonal pairs have almost identical values of reduction potentials.
Ered of Li+/Li = – 3.05 V
Ered of Be2+/Be = – 1.76 V (1.85V)
Ered of Mg2+/Mg = – 2.73 V
Ered of Al3+/Al = – 1.67 V
Important Diagonal Members
The diagonal relationship is prominent among the lighter members of the seconds and third periods
elements. Thus, Li (Lithium) of group IA shows the diagonal – relationship with Mg (Magnesium) of group
IIA, Be (Beryllium) of group IIA shows the diagonal – relationship with Al (Aluminium) of group IIIA, and B
(Boron) of group IIIA shows the diagonal – relationship with Si (Silicon) of group IVA respectively.
The important diagonal members in the periodic table are given below:
1) Li – Mg diagonal relationship
2) Be – Al diagonal relationship
3) B – Si diagonal relationship
4) Al – Zn diagonal relationship
5) Sc – Zr diagonal relationship
Conceptual Inorganic Chemistry Text Book -2-
Diagonal Relationship
Definition of Diagonal Relationship
The element of the 2nd period belonging to the IA, IIA and IIIA group is similar in behaviour to the element of
IIA, IIIA and IVA group respectively in the 3rd period. This Across-the-street similarity in properties between
two elements is called diagonal relationship.
The similarity in properties observed between two elements placed diagonally from
left to right in two adjacent periods and two adjacent groups in the periodic table is
known as the diagonal relationship.
OR
The close resemblance (similarity) of the properties between the first members of
second period with their diagonally opposite element of 3rd period belonging to next
group is called diagonal relationship.
The two elements showing similarities in properties are called Diagonal Neighbours or Diagonal Pair or
Bridge Elements. The diagonal members have similar atomic radii and polarising power i.e.
charge/size ratio is same.
Diagonal relationship is prominent between following pairs:
Reason of Diagonal Relationship
The diagonal relationship is attributed because of following reason:
1. Same charge density or Polarising Power or Ionic Potential (charge/size ratio)
2. Same Electronegativity
3. Same Reduction Potential
Conceptual Inorganic Chemistry Text Book -1-
, Dr Inam Ul Haq Jazbi Unit # 1 … Periodic Table (Diagonal Relationship)
1. Same charge density or Polarising Power or Ionic Potential
The diagonal pairs have almost same charge density (which is equal to charge/radius ratio) due to their
same ionic sizes. e.g.
Charge density of Be2+ = 2/0.31 = 6.45
Charge density of Al3+ = 3/0.5 = 6.0
2. Same Electronegativity
The diagonal pairs have almost same electronegativity. e.g. electronegativity of both Be and Al are same
(i.e. 1.5). Similarly electronegativity of Li is 1.0 and that of Mg is 1.2 and electronegativity of B is 2 and that
of Si is 1.8.
3. Same Reduction Potential
The diagonal pairs have almost identical values of reduction potentials.
Ered of Li+/Li = – 3.05 V
Ered of Be2+/Be = – 1.76 V (1.85V)
Ered of Mg2+/Mg = – 2.73 V
Ered of Al3+/Al = – 1.67 V
Important Diagonal Members
The diagonal relationship is prominent among the lighter members of the seconds and third periods
elements. Thus, Li (Lithium) of group IA shows the diagonal – relationship with Mg (Magnesium) of group
IIA, Be (Beryllium) of group IIA shows the diagonal – relationship with Al (Aluminium) of group IIIA, and B
(Boron) of group IIIA shows the diagonal – relationship with Si (Silicon) of group IVA respectively.
The important diagonal members in the periodic table are given below:
1) Li – Mg diagonal relationship
2) Be – Al diagonal relationship
3) B – Si diagonal relationship
4) Al – Zn diagonal relationship
5) Sc – Zr diagonal relationship
Conceptual Inorganic Chemistry Text Book -2-
