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 \(\mathrm{Li}, \mathrm{Mg}\) 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

Statements (2) and (4) are not true.

Step 1: Understand diagonal relationship

Diagonal relationship refers to the similarities between the properties of certain elements that are located diagonally across the periodic table. In this exercise, we focus on the diagonal relationship between Lithium (Li) and Magnesium (Mg). These elements can have comparable characteristics in terms of size, electronegativity, and ionic properties.

Step 2: Evaluate Statement (1)

Statement (1) claims that the polarizing powers of \(Li^{+} \)and \(Mg^{2+} \)are almost the same. Polarizing power is defined by the charge and size of an ion. Both \(Li^{+} \)and \(Mg^{2+} \)are small with similar charge densities; thus, this statement is generally true.

Step 3: Evaluate Statement (2)

Statement (2) says that like Li, Mg decomposes water very fast. \(Li \)reacts with water slowly, and \(Mg \)reacts with water very slowly, usually requiring heating to burn. Therefore, this statement is not true.

Step 4: Evaluate Statement (3)

Statement (3) suggests that \(LiCl \)and \(MgCl_2 \)are deliquescent. Deliquescent substances absorb moisture from the air. Both of these compounds are known to be deliquescent, making this statement true.

Step 5: Evaluate Statement (4)

Statement (4) indicates that like \(Li \), \(Mg \)readily reacts with liquid bromine at ordinary temperature. \(Li \)reacts slowly with bromine, while \(Mg \)reacts more slowly compared to alkali metals. As such, the readiness isn't as suggested, making this statement not true.

Key Concepts

Periodicity

Periodicity is a fundamental concept in chemistry that refers to the recurring trends in the properties of elements across the periodic table. These properties emerge from the arrangement of electrons in atoms as you move across periods (rows) and down groups (columns). Periodicity helps in predicting the behavior of elements in chemical reactions and understanding their physical characteristics.

A classic example of periodicity is the diagonal relationship. This relationship highlights that elements in different groups but similar configurations exhibit comparable properties. For instance, Lithium (Li) from Group 1 and Magnesium (Mg) from Group 2 show this relationship. They have comparable ionic radii and similar electronegativity, despite being situated diagonally across the periodic table. Knowing periodicity aids in not only learning about element similarities but also understanding why they behave in unique ways in reaction conditions.

Polarizing Power

Polarizing power relates to an ion's ability to distort the electron cloud of another ion, leading to a greater covalent character in bonds. It depends largely on the charge and size of an ion. A higher charge and smaller ionic radius provide greater polarizing power. Let's break it down for better understanding:

• A high polarizing power means the ion can easily distort another ion's electron cloud.
• Li⁺ and Mg²⁺ both possess small sizes and significant charges, thereby showing similar polarizing energies.
• This similarity enforces the diagonal relationship, where Li and Mg exhibit comparable behavior due to their polarizing capability.

Recognizing polarizing power allows chemists to predict how ions within compounds will behave, such as forming covalent rather than ionic bonds, which is vital for understanding bond formation and reactivity.

Chemical Reactivity

Chemical reactivity refers to the tendency of a substance to engage in chemical change. It involves how and why some substances react under given conditions, and it varies greatly amongst elements.

For instance, Lithium (Li) and Magnesium (Mg) display different reactivities to water. Li reacts slowly with water, while Mg reacts even more sluggishly, often requiring heating. This illustrates a real-world example where diagonal relationship fails to predict identical behavior under certain conditions.

• Li's reaction with bromine is slow, typical for alkali metals.
• Mg reacts less readily compared to alkali metals, indicating its lower reactivity.

Understanding the nuances of chemical reactivity helps in predicting reactions and selecting conditions for desired chemical processes in both academic experiments and industrial applications.

Deliquescent Substances

Deliquescent substances are materials that absorb moisture from the air until they dissolve in the absorbed water, forming a solution. This property is important in various applications, from industrial processes to household commodities.

• Substances like LiCl and MgCl₂ exhibit deliquescent properties.
• This happens because these salts have strong attractions to water molecules in the air, pulling them inward until the salts dissolve.

Deliquescence can be both beneficial and problematic. It helps in humidity control and some medical applications but can be problematic where a dry environment is necessary. Recognizing substances with deliquescent properties is crucial for handling and storing materials properly.