Problem 81 - A Complete Resource Book in Chemistry for JEE Main

Question

The electronic configuration of elements A, B and C are \([\mathrm{He}] 2 \mathrm{~s}^{1},[\mathrm{Ne}] 3 \mathrm{~s}^{1}\) and \([\mathrm{Ar}] 4 \mathrm{~s}^{1}\) respectively. Which one of the following order is correct for the first ionization potentials (in \(\mathrm{kJ} \mathrm{mol}\) ) of \(\mathrm{A}, \mathrm{B}\) and \(\mathrm{C}\) ? (a) \(\mathrm{A}>\mathrm{B}>\mathrm{C}\) (b) \(\mathrm{O}>\mathrm{B}>\mathrm{A}\) (c) \(\mathrm{B}>\mathrm{O}>\mathrm{A}\) (d) \(\mathrm{C}>\mathrm{A}>\mathrm{B}\)

Step-by-Step Solution

Answer

Option (a) \( \text{A} > \text{B} > \text{C} \).

Step 1: Understand Electronic Configurations

First, recognize that the electronic configurations given: \([\text{He}] 2s^1\), \([\text{Ne}] 3s^1\), and \([\text{Ar}] 4s^1\) correspond to the elements Lithium (Li), Sodium (Na), and Potassium (K) respectively. This is because these elements follow the after-mentioned noble gas configurations with an additional electron in the s-orbital of a higher energy level.

Step 2: Recall Periodic Trend of Ionization Energy

Ionization energy generally increases across a period (left to right) and decreases down a group (top to bottom) in the periodic table. This occurs because as we move across a period, the nuclear charge increases, whereas, moving down a group, the outermost electrons are further from the nucleus and are more easily removed.

Step 3: Compare Ionization Energies

Elements A (Li), B (Na), and C (K) are in the same group (Group 1, alkali metals). Thus, we expect the trend in their ionization energies to decrease down the group. Therefore, the order of ionization energies should be \( \text{Li} > \text{Na} > \text{K} \).

Step 4: Determine Correct Option

Knowing \( \text{Li} > \text{Na} > \text{K} \) corresponds to \( \text{A} > \text{B} > \text{C} \), option (a) is correct.

Key Concepts

Electronic Configuration

Electronic configuration refers to the arrangement of electrons in an atom's orbitals. It's like a code that tells us where each electron in an atom can be found. For example, the configuration \([ ext{He}] 2s^1\) means that after the noble gas helium (\( ext{He}\)), the next electron goes into the \(2s\) orbital. This specific configuration corresponds to lithium (Li).
Imagine it as a bus route where helium is the starting depot, and the \(2s^1\) orbital is the first stop. Similarly, \([ ext{Ne}] 3s^1\) for sodium (Na) and \([ ext{Ar}] 4s^1\) for potassium (K) can be broken down in the same way.
Understanding electronic configurations can help us predict many properties of the elements, such as ionization energy, which is vital in determining how an element will behave in chemical reactions. It offers insights into an atom's reactivity, stability, and position in the periodic table.

Periodic Trends

Periodic trends are the backbone of understanding the behavior of elements on the periodic table. A simple way to grasp this concept is to focus on two main trends: ionization energy and atomic size.

• **Ionization Energy** - This is the energy required to remove an electron from an atom. It typically increases as you move from left to right across a period because the atoms have more protons, pulling the electrons closer and making them harder to remove.
• **Atomic Size (or Radius)** - This increases as you move down a group because there are more energy levels, making the atom larger.

For elements like lithium, sodium, and potassium, which are in the same group, moving down the group means an increase in atomic size and a decrease in ionization energy. These trends are key for predicting and comparing how elements will interact in various chemical environments.

Alkali Metals

Alkali metals are a distinct group of elements located in Group 1 of the periodic table. This group includes lithium (Li), sodium (Na), and potassium (K), among others. They are characterized by having just one electron in their outermost s-orbital, which makes them highly reactive.
Some key characteristics of alkali metals are:

• **Highly Reactive** - Their single valence electron is relatively easy to remove, leading to high reactivity, especially with water.
• **Soft and Shiny** - Most alkali metals can be cut with a knife and have a shiny surface when freshly cut.
• **Low Density and Melting Points** - Compared to other metals, alkali metals have low densities and melting points.

Understanding alkali metals and their properties can help explain periodic trends. As you go down the group, the decrease in ionization energy makes elements like potassium more reactive than lithium. This is crucial for applications spanning from chemical industries to educational purposes, fostering a deeper understanding of fundamental chemistry concepts.