Problem 108 - A Complete Resource Book in Chemistry for JEE Main

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(l)

Step-by-Step Solution

Answer

(b) (2) and (4)

Step 1: Understanding the Diagonal Relationship

The diagonal relationship refers to the similarities between an element in the second period and an element in the third period which is one group to the right. Lithium ( ext{Li}) and Magnesium ( ext{Mg}) have such a relationship where they share similar properties despite being in different groups in the periodic table.

Step 2: Analyzing Statement (1)

Statement (1) claims that the polarizing powers of  ext{Li}^{+} and  ext{Mg}^{2+} are almost the same. Polarizing power is determined by the charge-to-size ratio. Both  ext{Li}^{+} and  ext{Mg}^{2+} have high polarizing power due to their small sizes and high charges, making this statement true.

Step 3: Analyzing Statement (2)

Statement (2) states that like  ext{Li},  ext{Mg} decomposes water very fast. In reality,  ext{Li} reacts with water very slowly, and  ext{Mg} also reacts with water at a slow rate, not rapidly. Thus, statement (2) is false.

Step 4: Analyzing Statement (3)

Statement (3) mentions that  ext{LiCl} and  ext{MgCl}_2 are deliquescent. This is true because both chloride salts absorb moisture from the air and dissolve in it.

Step 5: Analyzing Statement (4)

Statement (4) claims that like  ext{Li},  ext{Mg} reacts readily with liquid bromine at ordinary temperature.  ext{Li} reacts with bromine, but  ext{Mg} does not react as readily under ordinary conditions, which makes this statement false.

Step 6: Final Evaluation

Based on the analysis, Statement (2) and Statement (4) are identified as not true about the diagonal relationship between  ext{Li} and  ext{Mg}. Therefore, option (b) is the correct choice.

Key Concepts

Polarization Power

Polarization power refers to the ability of a cation to distort the electron cloud of an anion. This effect is most significant in cations with a high charge-to-size ratio. Such cations can pull the electron cloud of the anion closer, leading to greater distortion. This can cause ionic bonds to gain some covalent character.

Lithium (\(\text{Li}^+\)) and magnesium (\(\text{Mg}^{2+}\)) ions share a similar high polarizing power. The reason for this is their relatively small ionic radii combined with their charges. This results in a significant charge density around each ion. Due to their high polarizing powers, both \(\text{Li}^+\) and \(\text{Mg}^{2+}\) are capable of altering the electron clouds of nearby anions, leading to some chemical properties that can appear similar, despite being different elements.

Understanding polarization power helps explain why lithium and magnesium exhibit some similar chemical properties, though their behaviors are not identical in all scenarios.

Deliquescent Salts

Deliquescent salts are substances that absorb moisture from their surroundings until they dissolve in the absorbed water. This property is highly dependent on the hygroscopic nature of the salt, which describes its affinity for water.

Lithium chloride (\(\text{LiCl}\)) and magnesium chloride (\(\text{MgCl}_2\)) are classic examples of deliquescent materials. These salts can draw in moisture from the air, making them useful in applications that require drying agents or moisture control.

The phenomena of deliquescence are largely due to ionic compounds that are highly soluble and have a strong attraction to water molecules. When dissolved, they can lower the relative humidity of their environment, leading to practical uses in specific industrial and laboratory settings.

Reactivity with Water

The reactivity of elements with water varies widely across the periodic table, influenced by factors such as atomic structure, ionization energy, and metallic character. Lithium, a group 1 alkali metal, reacts with water slowly, forming lithium hydroxide and hydrogen gas.

Magnesium, part of the alkaline earth metals in group 2, similarly reacts with water, but the reaction is not as vigorous as that of group 1 elements, except under high temperatures or in steam. This is because magnesium forms a thin layer of magnesium hydroxide on its surface, which partially protects the metal from further reaction with water.

Understanding their water reactivities highlights the differences in chemical behavior between the two, despite their diagonal relationship.

Group Trends in Periodic Table

The periodic table is not just a collection of elements; it is an organized chart that depicts trends and relationships among different elemental families. Moving across a period from left to right, elements generally become less metallic, while descending a group increases metallic character.

Electronegativity typically increases across a period and decreases down a group, while atomic and ionic sizes increase as you move down a group. These trends help predict and explain the chemical properties and reactions of elements, such as the reactivity patterns observed with water and the phenomena like polarization and deliquescence.

The diagonal relationship highlights how some elements, despite being in different groups, share similar chemical behaviors due to a balance of these periodic trends, particularly regarding ionic radii and oxidation states.