Question

Pick out the statement(s) which is are not true about the diagonal relationship of \(\mathrm{Li}\) and \(\mathrm{Mg}\). (1) Polarizing powers of \(\mathrm{Li}^{+}\) and \(\mathrm{Mg}^{2+}\) are almost same. (2) Like Li, Mg decomposes water very fast. (3) \(\mathrm{LiCl}\) and \(\mathrm{MgCl}_{2}\) are deliquescent. (4) Like Li, Mg readily reacts with liquid bromine at ordinary temperature. (a) (1) and (4) (b) (2) and (4) (c) Only (2) (d) Only (1)

Short answer

(b) Statements 2 and 4 are not true.

Step-by-step solution

Understanding the Problem

We need to identify the statements about the diagonal relationship between Lithium (Li) and Magnesium (Mg) that are not true. There are four statements provided, and we need to assess each for its validity.

Review Diagonal Relationship

Li and Mg exhibit a diagonal relationship in the periodic table, meaning they share some similar properties due to their position when one moves diagonally across the table from Li in Group 1 to Mg in Group 2.

Evaluate Statement 1

Statement 1 claims that the polarizing powers of \(\mathrm{Li}^{+}\) and \(\mathrm{Mg}^{2+}\) are almost the same. This is true because both ions develop high charge density due to their small size and high charge.

Evaluate Statement 2

Statement 2 claims that Mg decomposes water very fast like Li. This is false. While Li reacts with water, Mg reacts with water only when it is hot, and not as vigorously as Li does.

Evaluate Statement 3

Statement 3 asserts that \(\mathrm{LiCl}\) and \(\mathrm{MgCl}_{2}\) are deliquescent. This is true as both salts absorb moisture from the atmosphere to a significant extent.

Evaluate Statement 4

Statement 4 says that Mg readily reacts with liquid bromine at ordinary temperature like Li. This is false. Lithium reacts with bromine, but Magnesium is less reactive with bromine under ordinary conditions.

Conclusion

Based on the evaluation, statements 2 and 4 are not true, as opposed to what the other statements suggest about the properties and reactivity of these elements.

Key concepts

Polarizing Power

Polarizing power refers to an ion's ability to distort an electron cloud. This key concept helps explain why some compounds have certain structural and chemical properties. Lithium ion (\(\mathrm{Li}^{+}\) ) and Magnesium ion (\(\mathrm{Mg}^{2+}\) ) share a significant resemblance in polarizing power. This is due to their high charge density.

• High charge density results in both ions having a large electrostatic potential.
• The smaller the ion and the higher the positive charge, the greater its polarizing power.

Because of these attributes, both \(\mathrm{Li}^{+}\) and \(\mathrm{Mg}^{2+}\) are highly effective at polarizing nearby electron clouds. This can lead to increased covalent character in primarily ionic compounds such as \(\mathrm{LiCl}\) and \(\mathrm{MgCl}_{2}\) , influencing their solubility and melting points.

Reactivity with Water

The reactivity of elements with water involves several atomic and molecular dynamics. Lithium is known for its vigorous reaction with water, forming lithium hydroxide and hydrogen gas. Magnesium's behavior with water, however, differs significantly.

• Unlike lithium, magnesium reacts with water only upon heating.
• At room temperature, magnesium barely reacts with water, forming a protective layer of hydroxide on its surface which inhibits vigorous reactions.

This variance in reactivity can be attributed to the different elemental characteristics of lithium and magnesium, such as their respective positions in the periodic table and energy requirements for reaction.

Deliquescence

Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water, forming a solution. Both \(\mathrm{LiCl}\) and \(\mathrm{MgCl}_{2}\) showcase this property prominently.

• These chloride compounds have strong ionic bonds that are capable of capturing water molecules from the surrounding environment.
• This ability to draw moisture originates from the highly polar nature of these salt's ions, aiding the breakdown of atmospheric water molecules.

Since deliquescent materials like \(\mathrm{LiCl}\) and \(\mathrm{MgCl}_{2}\) can absorb a significant amount of moisture, they are typically stored in airtight containers to prevent them from forming saturated solutions inadvertently.

Reaction with Halogens

The interaction of elements with halogens provides insight into their chemical reactivity and practical applications. Both lithium and magnesium react with halogens, but their reaction conditions differ.

• Lithium reacts readily with liquid bromine at ordinary temperatures, forming lithium bromide.
• Conversely, magnesium requires elevated conditions to react effectively with bromine; it does not readily react at room temperature due to a more robust metallic bonding.

Understanding these elemental reactions with halogens is important in chemistry because it highlights the contrasting behavior among closely related elements like lithium and magnesium, reinforcing the concept of diagonal relationships in the periodic table.