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

Question

The electronic configuration of elements \(\mathrm{A}, \mathrm{B}\) and \(\mathrm{C}\) 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) \(O>B>A\) (c) \(\mathrm{B}>\mathrm{O}>\mathrm{A}\) (d) \(C>A>B\)

Step-by-Step Solution

Answer

The correct order is (a) \( \mathrm{A} > \mathrm{B} > \mathrm{C} \).

Step 1: Understand First Ionization Potential

The first ionization potential is the energy required to remove the outermost electron from a gaseous atom. It generally increases across a period and decreases down a group in the periodic table.

Step 2: Analyze Electronic Configurations

Elements - \( \mathrm{A} : [\mathrm{He}] 2 \mathrm{~s}^{1} \) is Lithium (\( \mathrm{Li} \)).- \( \mathrm{B} : [\mathrm{Ne}] 3 \mathrm{~s}^{1} \) is Sodium (\( \mathrm{Na} \)).- \( \mathrm{C} : [\mathrm{Ar}] 4 \mathrm{~s}^{1} \) is Potassium (\( \mathrm{K} \)).These elements are all part of Group 1 (alkali metals).

Step 3: Evaluate Periodic Trends

For alkali metals, as we move down the group from Lithium to Potassium, the first ionization potentials decrease. This is due to increasing atomic size and electron shielding, which make it easier to remove the outermost electron.

Step 4: Establish Ionization Potential Order

For \( \mathrm{Li}, \mathrm{Na}, \mathrm{K} \), the order of first ionization potential, from highest to lowest, is Lithium > Sodium > Potassium.

Step 5: Identify Correct Answer

Comparing the order with the given options:- Option (a) \( \mathrm{A} > \mathrm{B} > \mathrm{C} \) matches our derived order.Thus, option (a) is the correct answer.

Key Concepts

Electronic Configuration

Electronic configuration is the distribution of electrons in an atom's electron shells and subshells. Each element has a unique electronic configuration that determines its chemical properties. For example, the electronic configurations provided in the original exercise:

• Element A: \([ ext{He}] 2s^1\), representing Lithium (Li)
• Element B: \([ ext{Ne}] 3s^1\), representing Sodium (Na)
• Element C: \([ ext{Ar}] 4s^1\), representing Potassium (K)

Understanding these configurations helps predict how elements will react chemically. Electrons fill subshells in an atom according to the Aufbau principle, which follows a specific order based on increasing energy levels. The filled subshells define each element's place in the periodic table and its behavior in chemical reactions.

Periodic Table Trends

The periodic table is organized in such a way that reveals patterns or trends in the properties of elements. One key trend is the ionization potential, which generally increases across a period and decreases down a group. This trend is influenced by:

• Atomic radius: As you move across a period, atoms tend to have smaller radii due to increased nuclear charge pulling electrons closer.
• Energy levels: Moving down a group, additional electron shells increase the atomic size, making it easier to remove an outer electron.

Ionization energy is crucial for understanding reactivity and stability of elements. Elements with high ionization energies are less likely to lose electrons easily, whereas elements with lower ionization energies are more reactive, particularly in forming ionic compounds.

Alkali Metals

Alkali metals, found in Group 1 of the periodic table, include elements like lithium (Li), sodium (Na), and potassium (K). These metals are characterized by having one electron in their outermost shell, making them very reactive. Key properties of alkali metals include:

• Low ionization energy: Their single valence electron can be easily removed.
• Reactiveness: They react vigorously with water and halogens to form alkali hydroxides and halide salts, respectively.
• Softness: They are generally soft metals that can be cut with a knife.

Understanding these metals' electronic configurations and their position in the periodic table helps us predict their chemical behavior, especially their tendency to form +1 ions.

Electron Shielding

Electron shielding occurs when inner shell electrons reduce the attractive force between the nucleus and the outer electrons. This phenomenon affects the ionization energies of elements. As you move down a group, like in the case of alkali metals:

• Increased shielding: More inner shells mean greater electron shielding.
• Reduced effective nuclear charge: Outer electrons feel less pull from the nucleus because of the inner shells' blocking effect.

These effects lower the ionization energy, making it easier for the outermost electron to be removed. Consequently, elements with substantial electron shielding, such as potassium compared to lithium, have lower ionization potentials. This concept is key to understanding why alkali metals become more reactive as you move down the group.