Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of atomic size of \(\mathrm{C}, \mathrm{N}, \mathrm{P}\) and \(\mathrm{S}\) follows the order (a) \(\mathrm{C}<\mathrm{N}<\mathrm{S}<\mathrm{P}\) (b) \(C

Step-by-Step Solution

Answer

Option (d): \(\mathrm{N}<\mathrm{C}<\mathrm{P}<\mathrm{S}\) is the correct order.

Step 1: Understanding Atomic Size

Atomic size, also known as atomic radius, typically increases as you move down a group in the periodic table and decreases as you move from left to right across a period. As a result, elements in the same group have larger atomic sizes as you move down the group, and elements in the same period have smaller atomic sizes as you move from left to right.

Step 2: Placing Elements in the Periodic Table

First, identify where each element is located in the periodic table: Carbon (C) and Nitrogen (N) are in Period 2, with Carbon located to the left of Nitrogen. Phosphorus (P) and Sulfur (S) are in Period 3, with Phosphorus located to the left of Sulfur.

Step 3: Comparing Atomic Sizes within Periods

Within Period 2, Nitrogen has a smaller atomic radius than Carbon because atomic size decreases from left to right. Similarly, within Period 3, Phosphorus has a larger atomic radius than Sulfur for the same reason.

Step 4: Comparing Atomic Sizes between Periods

Now, compare elements from different periods. Elements in Period 3, such as Phosphorus and Sulfur, generally have larger atomic sizes than those in Period 2, such as Carbon and Nitrogen, because atomic size increases as you move down a group.

Step 5: Arranging Elements by Atomic Size

Based on the above steps, arrange the elements from smallest to largest atomic size: Nitrogen (smallest) < Carbon < Sulfur < Phosphorus (largest). This corresponds to option (d).

Key Concepts

Periodic Table

The periodic table is a tabular arrangement of chemical elements, organized by their atomic number, electron configurations, and recurring chemical properties. The table's main part consists of rows called periods and columns called groups. Each element is placed according to its atomic structure, giving us a systematic way to compare and contrast the behavior and properties of the elements. Key features of the periodic table include:

• Metals are generally found on the left side, while nonmetals are on the right.
• Groups are vertical columns that classify elements with similar chemical behaviors.
• Periods are horizontal rows that represent elements with the same number of electron shells.

For example, in the periodic table, carbon (C) and nitrogen (N) are found in Period 2, while phosphorus (P) and sulfur (S) are in Period 3. This arrangement is pivotal for predicting and understanding the properties, including atomic size, of the elements.

Atomic Radius

The atomic radius is a measure of the size of an atom, typically the distance from the nucleus to the boundary of the surrounding cloud of electrons. The atomic radius can vary under different definitions, including:

• Covalent radius: Typically used for elements that form covalent bonds.
• Ionic radius: Applied when atoms are part of ionic compounds.
• Metallic radius: Relevant for metals in metallic bonds.

In general, as electrons are added to the electron cloud, repulsion among the electrons increases, usually leading to a larger radius. However, within a period from left to right, the atomic radius decreases due to increasing nuclear charge pulling the electrons closer to the nucleus. For instance: - In Period 2, Carbon's (C) atomic radius is larger than Nitrogen (N)'s. - In Period 3, Phosphorus (P) has a larger atomic radius than Sulfur (S). Understanding atomic radius is essential because it affects the reactivity and properties of an element.

Group and Period Trends

The periodic table reveals trends in atomic size both within groups and across periods. These trends help chemists predict an element's behavior based on its position. **Trends within Groups:**

• Atomic size increases down a group as additional electron shells are added, moving elements further from the nucleus.
• This is why Phosphorus (P) and Sulfur (S) have larger atomic sizes compared to Carbon (C) and Nitrogen (N).

**Trends across Periods:**

• Atomic size decreases from left to right across a period. This trend occurs because as the atomic number increases, there is greater nuclear charge attracting the electron cloud inward, reducing the size.
• Thus, within the same period, Nitrogen (N) is smaller than Carbon (C) in Period 2, and Sulfur (S) is smaller than Phosphorus (P) in Period 3.

Understanding these trends allows students to arrange elements based on atomic size logically. For instance, the exercise shows the order: Nitrogen < Carbon < Sulfur < Phosphorus, illustrating the influence of both group and period trends.