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

Statements (a), (b), and (c) are true.

Step 1: Identify IA Elements

IA elements, also known as alkali metals, include Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), and Francium (Fr). We will refer to these elements as we assess the truth of each statement.

Step 2: Evaluate Statement (a)

The statement is: Na is regarded as a typical representative element. Sodium is one of the most commonly encountered alkali metals, used often to illustrate the properties of IA elements. Thus, this statement is true.

Step 3: Evaluate Statement (b)

The statement is: Basic character of their oxides increases down the group. As we move down Group 1, the metals become more reactive, leading to stronger basic oxides. Therefore, this statement is true.

Step 4: Evaluate Statement (c)

The statement is: Among alkali metal ions, the degree of hydration is the highest in Li. Lithium ions are smaller and therefore have a higher charge density, which allows them to attract more water molecules compared to larger alkali metal ions. Hence, this statement is true.

Step 5: Evaluate Statement (d)

The statement is: In general electron affinity values decrease from top to bottom in the group. Electron affinity generally decreases down the group as the atomic size increases, making it less favorable for the atom to gain an electron. Thus, this statement is not true for alkali metals as they tend to have less pronounced electron affinity trends.

Key Concepts

Alkali Metals

The IA elements in the periodic table are known as alkali metals. These include Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), and Francium (Fr). These metals form the first group of the periodic table and are known for their distinct properties.
The term 'alkali' originates from their ability to form alkaline solutions when they react with water. These metals are highly reactive, especially with water, and they react so vigorously that they must be stored in oil to prevent contact with moisture in the air.

• Alkali metals are soft and can often be cut with a knife.
• They have a shiny, silvery appearance but tarnish quickly when exposed to air.
• These metals have low densities, with some like lithium floating on water.
• They have low melting and boiling points compared to most other metals.

Understanding alkali metals is crucial because their reactivity and characteristics serve as a foundation for exploring other elements in the periodic table.

Basic Character of Oxides

The oxides of alkali metals exhibit basic properties, and this basicity becomes more pronounced as you move down the group. When alkali metals react with oxygen, they form oxides which are typically strong bases.
For example, Sodium ( Na) reacts with oxygen to form Sodium Oxide ( Na₂O), which is a strong base in water, forming sodium hydroxide ( NaOH).
As we progress down the group, these oxides form stronger bases for several reasons:

• The metals become more electropositive, enhancing their ability to donate electrons.
• The oxides react more readily with water to form hydroxides, which are strong bases.

Consequently, elements lower in the group, such as potassium and cesium, form oxides that are even stronger bases than those of lithium and sodium. This trend is a reflection of the increasing metallic character of the alkali metals.

Degree of Hydration

In the context of alkali metal ions, the degree of hydration refers to the number of water molecules surrounding an ion. This is largely influenced by the size and charge density of the ion.
Lithium ions ( Li⁺) have the highest degree of hydration among alkali metal ions. This happens because:

• Lithium ions are the smallest in size among the group.
• Their high charge density allows them to exert a strong attractive force on surrounding water molecules.

The ability of lithium ions to attract more water is what gives rise to greater hydration. This contrasts with larger ions like cesium ( Cs⁺), which have a lower charge density and therefore a lower degree of hydration. This concept is essential for understanding reactions involving aqueous solutions and the solubility of compounds.

Electron Affinity Trends

Electron affinity refers to the amount of energy released when an atom in the gaseous state accepts an electron. In the case of alkali metals, electron affinity generally does not follow the usual trend seen in other groups.
Typically, electron affinity decreases as one moves down a group due to increasing atomic size and shielding effect, which makes it less energetically favorable to add an electron. However, for alkali metals, this trend is not as pronounced or relevant:

• Alkali metals are more inclined to lose an electron and form positive ions, given their single electron in the outermost shell.
• Their low electron affinity reflects the preferred loss rather than gain of an electron.

Understanding this concept is vital, as it sheds light on the unique chemical behavior of alkali metals. They are more reactive in losing electrons rather than gaining them, which is opposite to what electron affinity describes for many other elements.