An element’s chemical properties are closely associated with the atoms in terms of size. A diagonal relationship exists between specific pairings of diagonally adjacent nodes in the second and third periods of the atomic numbers. These pairings share several common features. For example, in the periodic table of elements, the diagonal relationship of lithium and magnesium is opposite in the second and third periods, and they have many similar properties.

Definition of a Diagonal Relationship

In the periodic table, some elements have a diagonal relationship. These elements lie diagonally adjacent to one another in the periodic table’s second and third rows from the first 20 elements.

The properties of diagonal elements are frequently identical, as seen when moving down the group and from left to right. It is evident among the lighter ones. Thus, the pairs that exhibit a diagonal relationship are as follows:

• Lithium (Li) from group IA with Magnesium (Mg) from group IIA
• Beryllium (Be) from group IIA with aluminium (Al) from group IIIA
• Boron (B) from group IIIA with silicon (Si) from group IVA
• Carbon (C) from group IVA with phosphorus (P) from group VA

Cause of Lithium-Magnesium Diagonal Relationship

Magnesium and lithium have a diagonal relationship.

• Despite its small size, lithium is distinct from all other alkali metals yet similar to magnesium due to its similar size. The equal size of the ions results in the formation of the diagonal relationship of lithium with magnesium.
• As we proceed down through a periodic table group, atom size increases gradually.
• Atom size decreases as we continue across a periodic table’s period.
• A diagonal relation was observed between specific pairings of diagonally neighbouring elements in the second and third periods (first 20 elements).

Anomalous Lithium Behavior and Diagonal Relationship with Magnesium

In many ways, lithium resembles magnesium, the element on the other side of the periodic table from it in group 1, yet it also has certain unique features of its own.

This unusual behavior of lithium is because it is an ion with an extremely small size.

Due to the Li+ ion’s small size, it has a high charge density. Thus, lithium-ion is the most polarising of all alkali metal ions and greatly distorts the negative ion. As a result, the Li+ ion exhibits an unusually high propensity for solvation and the creation of covalent bonds. Additionally, it is worth noting that the polarising strength of the Li+ ion is comparable to that of the Mg2+ ion, implying that the two elements (located diagonally on the periodic table) share similar features.

The resemblances between these periodic table elements can be explained by the elements’ electropositive nature and polarising activity.

(i) While the elements’ electropositive nature grows as we move down a group, it diminishes as we move from left to right across a period. As a result, the electropositive properties of diagonally opposite elements are identical.

(ii) Moving down a group increases the size and decreases polarising power; moving across a group from left to right raises the charge on the ions and reduces size, increasing polarising capacity. As a result, the second period’s element has the same polarising effect as the third period’s lower right element. As a result, their compounds should exhibit similar properties. There are some fascinating similarities between the types and strengths of bonds formed between atoms of different valencies and the properties of their compounds.

Diagonal Relationship of Lithium with Magnesium

Some factors that lithium and magnesium have in common are:

• The electronegativities of lithium and magnesium are nearly identical.
• Li and Mg are both covalent.
• Compared to other alkali metals, lithium seems much harder and lighter. Compared to magnesium, lithium has a similar hardness.
• Lithium has a boiling point of 1603 K, comparable to the boiling point of magnesium at 1373 K.
• These hydroxides of Li and Mg are weak bases that readily break down when heated.

2LiOH→LiO2 + H2O

Mg (OH) 2→MgO + H2O

• Aqueous hydrates of lithium chloride and magnesium chloride crystallize after they deliquesce. Additionally, these chlorides dissolve in ethanol.
• The ions Li+ and Mg2+ have a high degree of hydration.
• In water, both Li and Mg are weakly soluble in hydroxide, carbonate, phosphate, and fluoride.
• Neither lithium nor magnesium produces solid bicarbonates.
• LiCl and MgCl are covalently soluble in ethanol.
• Ethanol is a highly soluble solvent for lithium and magnesium perchlorate.

Conclusion

The positioning of certain elements in the periodic table with similar properties across a diagonal is called a diagonal relationship. Lithium lies in the first and second periods in the current periodic table. It is similar to magnesium, classified as a second group and third-period element. Lithium and magnesium are both compact and have a high charge density. Li has an electronegativity of 1.0, while Mg has a value of 1.2. They are both low and nearly identical. The ionic radii of these two elements are similar. Because of this, they exhibit similarities which are considered a diagonal relationship between the first element of a group and the second element of the next higher group.