Problem 32 - A Complete Resource Book in Chemistry for JEE Main

Question

Ionic radius (in A) of \(\mathrm{As}^{3+}, \mathrm{Sb}^{3+}\) and \(\mathrm{Bi}^{3+}\) follow the order (a) \(\mathrm{As}^{3+}>\mathrm{Sb}^{3+}>\mathrm{Bi}^{3+}\) (b) \(\mathrm{Sb}^{3+}>\mathrm{Bi}^{3+}>\mathrm{As}^{3+}\) (c) \(\mathrm{Bi}^{3+}>\mathrm{As}^{3+}>\mathrm{Sb}^{3+}\) (d) \(\mathrm{Bi}^{3+}>\mathrm{Sb}^{3+}>\mathrm{As}^{3+}\)

Step-by-Step Solution

Answer

The correct order is (d) \(\mathrm{Bi}^{3+} > \mathrm{Sb}^{3+} > \mathrm{As}^{3+}\).

Step 1: Understand Ionic Radius

Ionic radius is a measure of the size of an ion. As we move down a group in the periodic table, the ionic radius generally increases because additional electron shells are added.

Step 2: Position in the Periodic Table

Arsenic (As), Antimony (Sb), and Bismuth (Bi) are all in the same group (Group 15) in the periodic table. As we move from As to Sb to Bi, each element has more electron shells.

Step 3: Determine the Order of Ionic Radius

Since more electron shells generally mean a larger ionic radius, the ionic radius should increase from As to Sb to Bi. Therefore, the correct order based on ionic radius is \(\mathrm{As}^{3+} < \mathrm{Sb}^{3+} < \mathrm{Bi}^{3+}\).

Step 4: Match Order to Options

The order \(\mathrm{As}^{3+} < \mathrm{Sb}^{3+} < \mathrm{Bi}^{3+}\) corresponds to option (d) \(\mathrm{Bi}^{3+} > \mathrm{Sb}^{3+} > \mathrm{As}^{3+}\).

Key Concepts

Periodic Table Group 15

Arsenic (As), Antimony (Sb), and Bismuth (Bi) are elements that belong to Group 15 of the periodic table. These elements, also known as the nitrogen group, are characterized by having five electrons in their outermost electron shell. This common trait among Group 15 elements affects their chemical properties and trends.

Group 15 elements range from nonmetals, like nitrogen, to metals, like bismuth, as you move down the group. This transition highlights a shift from nonmetallic to metallic properties.

Key points about Group 15:

• Valency: They typically exhibit a valency of -3 when forming ions, due to their tendency to gain three electrons to attain a stable octet configuration.
• Electron Configuration: They share a general electron configuration of ns²np³.
• Trends: Various properties such as electronegativity, reactivity, and ionization energy show noticeable trends as you move from the top of the group to the bottom.

Electron Shells

Each period in the periodic table signifies an increase in the principal electron shell. Diagrams of electron configurations often present these shells as concentric circles around the nucleus.

Moving down Group 15 from arsenic to bismuth adds additional electron shells:

• Arsenic has four electron shells.
• Antimony has five electron shells.
• Bismuth includes six electron shells.

As a result, elements with more electron shells generally have larger atomic and ionic radii.

Despite the larger number of electron shells increasing size, the outermost electrons are less tightly bound due to increased distance from the nucleus and shielding by inner shells.

Key considerations for electron shells:

• Core electrons (inner shells) shield outer electrons from the attractive force of the nucleus.
• This shielding effect results in more significant electron screening and larger ionic radii.
• As additional electron shells are added, atoms within the same group grow bigger.

Ionic Size Trend

The trend of ionic size within a group in the periodic table is significantly influenced by the number of electron shells. As you move down Group 15, from arsenic to bismuth, the ionic size increases. This increase is due to the added electron shells with each subsequent element in the group.

Understanding the influence of ionic size trend:

• Larger ionic size occurs because each additional electron shell positions the valence shell further from the nucleus.
• This creates a decreased effective nuclear charge perceived by the valence electrons due to enhanced shielding.
• Bismuth (\(\mathrm{Bi}^{3+}\)) will thus have the largest ionic radius among the three ions (\(\mathrm{As}^{3+}, \mathrm{Sb}^{3+}, \mathrm{Bi}^{3+}\)).

The behavior of ionic size can help predict reactivity and the types of bonds an element might form during chemical reactions, acting as a useful property for chemists exploring trends across different groups.