The Modern periodic table is based on the law- “The chemical properties of elements are a periodic function of their atomic number.” We have studied the trends along with the group and across the period. The pairs of diagonally adjacent elements in the second and third periodic table are called Diagonal Relationship of the Elements

But have you ever wondered why there are similarities between Lithium and Magnesium, Beryllium and Aluminium, etc., in the periodic table diagonally? This article will teach you about the Diagonal relationship in the periodic table. Read on to find more.

What is Diagonal Relationship?

The diagonal relationship exists between certain elements in the periodic table. These elements are placed diagonally adjacent in the second and third row of the periodic table, among the first twenty elements.
The diagonal elements usually show similarities in their properties, which is exhibited on moving from left to right and down the group in the periodic table. It is prominent among the lighter members. Thus the pairs that show diagonal relationship are-

1. Lithium \(\left( {{\rm{Li}}} \right)\) of group IA and Magnesium \((\text{Mg})\) of group IIA,
2. Beryllium \((\text{Be})\) of group IIA and Aluminium \((\text{Al})\) of group IIIA,
3.  Boron \((\text{B})\) of group IIIA and Silicon \((\text{Si})\) of group IVA respectively.
4. Carbon \((\text{C})\) of group IVA and Phosphorus \((\text{P})\) of group VA respectively.

Reason for Diagonal Relationship

The diagonal relationship exists because of how different atomic properties vary between groups and periods in the periodic table.

OR
Due to the polarizing power of the diagonally placed elements, the polarizing power increases due to the increase in charge of the ions, whereas the ionic size decreases. On moving down the group, the ionic size decreases due to which polarizing power decreases. On moving diagonally, these effects cancel each other to some extent.

Diagonal Relationship Between Lithium and Magnesium

Some of the similarities between Lithium and Magnesium are as follows:

1. Both \(\text{Li}\) and \(\text{Mg}\) have almost the same electronegativities.
2. Both \(\text{Li}\) and \(\text{Mg}\) have covalent character.
3. They are harder and lighter than other elements in their groups.
4. The hydroxides \(\text{LiOH}\) and \(\text{Mg(OH)}_2\) are weak bases and decompose on heating.
   \({\text{2LiOH}} \to {\text{Li}}{{\text{O}}_{\text{2}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}}\)
   \({\text{Mg}}{\left({{\text{OH}}}  \right)_{\text{2}}} \to {\text{MgO + }}{{\text{H}}_{\text{2}}}{\text{O}}\)
5. Chlorides \(\text{LiCl}\) and \(\text{MgCl}_2\) are deliquescent and crystallize from an aqueous hydrate. Also, these chlorides are soluble in ethanol.
6. The hydroxide, carbonate, phosphates and fluorides of both \(\text{Li}\) and \(\text{Mg}\) are sparingly soluble in water.

Diagonal Relationship Between Beryllium and Aluminium

Some of the similarities between Beryllium and Aluminium are as follows:

1. Both elements are metals and tend to form covalent bonds.
2. Both elements form compounds that have a low melting point and are soluble in the organic solvent.
3. The oxides and hydroxides of both elements are amphoteric in nature.
4. Both beryllium and aluminium react very slowly with dilute hydrochloric acid to liberate hydrogen gas.
5. Both \(\text{BeCl}_2\) and \(\text{AlCl}_3\) act as a catalyst in Friedel -Crafts reaction.
6. Both react with the base \(\text{NaOH}\) to liberate hydrogen gas forming beryllates and aluminates.
   \({\text{Be + 2NaOH}} \to {\text{N}}{{\text{a}}_{\text{2}}}{\text{Be}}{{\text{O}}_{\text{2}}}{\text{ + }}{{\text{H}}_{\text{2}}}\)
   \({\text{2Al + 6NaOH}} \to {\text{N}}{{\text{a}}_{\text{3}}}{\text{Al}}{{\text{O}}_{\text{3}}}{\text{ + 3}}{{\text{H}}_{\text{2}}}\)

Diagonal Relationship Between Boron and Silicon

Some of the similarities between Boron and silicon are as follows:

1. Both \(\rm{B}\) and \(\rm{Si}\) are non-metals.
2. Both the elements have high melting and boiling point.
3. Both elements have almost the same densities and low atomic volumes.
4. Both the elements form covalent compounds.
5. Both the elements exist in the amorphous and crystalline state.
6. Both the elements exhibit allotropy.

Diagonal Relationship Between Carbon and Phosphorus

Some of the similarities between carbon and phosphorus are as follows:

1. Both carbon and phosphorus exhibit allotropes.
2. Both carbon and phosphorus show almost the same electronegativities.
3. Both form strong pi and sigma bonds.
4. Like carbon, phosphors can also form triple bonds.

Summary

This article teaches that the diagonal elements placed in the second and third rows show similarities in their properties. Also, we studied the causes for such similarity and the pair of elements that show this relationship.

FAQs

Q.1. What is the diagonal relationship?
Ans: The diagonal relationship is said to exist between certain elements in the periodic table. These elements are placed diagonally adjacent in the second and third row of the periodic table, among the first twenty elements. The diagonal elements usually show similarities in their properties, which is exhibited on moving from left to right and down the group in the periodic table. It is prominent among the lighter members. Thus the pairs that show diagonal relationship are- Lithium \((\text{Li})\) of group IA and Magnesium \((\text{Mg})\) of group IIA, Beryllium \((\text{Be})\) of group IIA and Aluminium \((\text{Al})\) of group IIIA, Boron \((\text{B})\) of group IIIA and Silicon \((\text{Si})\) of group IIA respectively.

Q.2. What causes the diagonal relationship?
Ans: The diagonal relationship occurs due to the polarizing power of the diagonally placed elements- On moving along the period, the polarizing power increases due to the increase in charge of the ions, whereas the ionic size decreases. On moving down the group, the ionic size decreases due to which polarizing power decreases. On moving diagonally, these effects cancel each other to some extent.

Q.3. Give examples of diagonal relationship.
Ans: The pairs that show diagonal relationship are- Lithium \((\text{Li})\) of group IA and Magnesium \((\text{Mg})\) of group IIA, Beryllium \((\text{Be})\) of group IIA and Aluminium \((\text{Al})\) of group IIIA, Boron \((\text{B})\) of group IIIA and Silicon \((\text{Si})\) of group IIA respectively.

Q.4. Do Boron and silicon show diagonal relationship?
Ans: Yes, Boron \((\text{B})\) of group IIIA and Silicon \((\text{Si})\) of group IIA show the diagonal relationship.

Q.5. What is the diagonal effect?
Ans: Some elements of period 2 and period 3 have the tendency to not have characteristics of their own family, but characteristics close to the element that is the right one and down one. Hence, this is the diagonal effect.

Q.6. What is the cause of diagonal relationship?
Ans: The diagonal relationship is formed due to almost similar size of the ions.

We hope this article on the Diagonal Relationship is helpful to you. If you have any questions about this article or, in general, about the Diagonal Relationship, ping us through the comments section, and we will get back to you as soon as possible.