Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement (s) is/are true for IA elements? (a) Na is regarded as a typical representative element. (b) Basic character of their oxides increases down the group. (c) Among alkali metal ions degree of hydration is the highest in Li. (d) In general electron affinity values decrease from top to bottom in the group.

Step-by-Step Solution

Answer

All statements (a), (b), (c), and (d) are true.

Step 1: Analyze Statement (a)

Statement (a) claims that Na is regarded as a typical representative element. Sodium (Na) is an alkali metal and is indeed considered a typical representative of Group 1 (IA) elements due to its well-known properties that are characteristic of the group. **Statement (a) is True.**

Step 2: Analyze Statement (b)

Statement (b) claims that the basic character of their oxides increases down the group. In the alkali metal group (IA), the basicity of oxides indeed increases as you move down the group due to the increasing strength of the metal-oxygen ionic bond. **Statement (b) is True.**

Step 3: Analyze Statement (c)

Statement (c) asserts that among alkali metal ions, the degree of hydration is highest in Li. Lithium (Li) ions are small and highly charged, leading to a strong interaction with water molecules, resulting in a high degree of hydration. **Statement (c) is True.**

Step 4: Analyze Statement (d)

Statement (d) claims that, in general, electron affinity values decrease from top to bottom in the group. Electron affinity typically decreases down a group as atomic size increases, making it harder to add an electron. However, group 1 elements do not favor gaining electrons, so their general trend in electron affinity is not easily predictable. Nevertheless, **statement (d) is generally considered to be True**, but not strongly evident for alkali metals.

Key Concepts

Alkali Metals

Alkali metals represent the Group 1 elements on the periodic table. This includes elements like Lithium (Li), Sodium (Na), and Potassium (K). They are characterized by having a single electron in their outermost shell, making them highly reactive. Due to their high reactivity with water, they are often stored in oil to prevent exposure to moisture.

• Alkali metals are soft and can be cut with a knife.
• They have low densities, with lithium being the least dense solid element.
• These metals also have low melting and boiling points compared to most other metals.

Their reactivity increases as you move down the group, meaning potassium is more reactive than sodium, for example. This increase in reactivity is due to the ease with which these metals can lose their outer electron. Understanding these properties helps in predicting both chemical behavior and suitable storage conditions.

Basicity of Oxides

The basicity of alkali metal oxides tends to increase as you move down Group 1. This happens because as the metals get larger, their oxides become more ionic.
This means the oxygen atoms are more strongly negatively charged, thus more ready to react with protons ( H^+ ) to form OH^- in water, making the solution more basic.

• Lithium oxide (Li₂O) is less basic compared to sodium oxide (Na₂O) and potassium oxide (K₂O).
• Cesium oxide (Cs₂O) is one of the most basic oxides among the alkali metals.

Alkali metal oxides can react with water to produce corresponding hydroxides, which are also strong bases. Understanding this property is essential when considering industrial applications and reactivity with acids.

Degree of Hydration

The degree of hydration refers to how well ions attract and hold onto water molecules. Among the alkali metals, lithium ions ( Li^+ ) have the highest degree of hydration.
This is because lithium ions are the smallest, allowing them to pack a significant amount of charge into a small volume, creating a strong electric field around them.
As a result, they can attract and stabilize more water molecules compared to larger ions like potassium ( K^+ ).
This property is important for understanding solubility and the behavior of alkali metals in aqueous solutions.

• The high degree of hydration of Li^+ leads to unique properties, making lithium salts often less soluble in organic solvents.
• This influences industrial processes, such as battery production and drug formulation.

Electron Affinity

Electron affinity measures an atom's tendency to attract an electron. For Group 1 elements, electron affinity generally decreases from lithium to cesium.
However, these elements have low electron affinities overall because they tend to lose electrons rather than gain them.
This trend occurs because the added electron enters an outer shell that is progressively farther from the nucleus as you move down the group.

• As the atomic radius increases, the added electron feels less attraction from the nucleus.
• This makes it harder for large alkali atoms to hold onto extra electrons.

Understanding electron affinity is crucial for predicting how elements will behave in reactions, especially in gaining and losing electrons. It's also essential for explaining the stability of ions in various chemical compounds.