Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

The configuration of inert gas with smallest size and highest IE is (a) \(1 \mathrm{~s}^{2}\) (b) \(1 \mathrm{~s}^{2} 2 \mathrm{~s}^{2} 2 \mathrm{p}^{6}\) (c) \(1 s^{2} 2 s^{2} 2 p^{5}\) (d) \(1 \mathrm{~s}^{2} 2 \mathrm{~s}^{2} 2 \mathrm{p}^{6} 3 \mathrm{~s}^{2} 3 \mathrm{p}^{6}\)

Step-by-Step Solution

Answer

The configuration is (a) \(1s^2\) for Helium.

Step 1: Understand the Given Atomic Configurations

The given configurations are those of elements or ions that resemble noble gases (inert gases) due to their full valence shell. Let's identify these configurations: \(1s^2\) is Helium, \(1s^2 2s^2 2p^6\) is Neon, \(1s^2 2s^2 2p^5\) is not a full shell configuration (it's actually identical to Fluorine), and \(1s^2 2s^2 2p^6 3s^2 3p^6\) is Argon.

Step 2: Identify the Smallest Atomic Size

The atomic size typically decreases across a period because the increasing nuclear charge pulls the electron cloud closer to the nucleus. Therefore, among the inert gases Helium \((1s^2)\), Neon \((1s^2 2s^2 2p^6)\), and Argon \((1s^2 2s^2 2p^6 3s^2 3p^6)\), Helium has the smallest atomic size as it is furthest to the right in the first period.

Step 3: Determine the Highest Ionization Energy

Ionization energy generally increases across a period and decreases down a group. Due to high effective nuclear charge and minimal electron shielding, Helium has the highest ionization energy within inert gases, as it has fewer electron shells compared to Neon and Argon.

Step 4: Conclude Based on Analysis

Given that Helium \((1s^2)\) has both the smallest atomic size and the highest ionization energy among the listed inert gas configurations, it is the correct choice.

Key Concepts

Noble Gases

Noble gases, often referred to as inert gases, are a unique group of elements on the periodic table. They reside in Group 18 and are characterized by their full valence electron shells. This configuration makes them exceptionally stable and chemically non-reactive under normal conditions.

• The family includes Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn).
• Their completed outer electron shells mean they do not readily form bonds with other elements.
• Their lack of reactivity is why they are termed 'noble', similar to the noble metals that don't easily engage in chemical reactions.

These elements have fascinating applications due to their inertness, ranging from helium-filled balloons to neon lights. Each noble gas has distinct uses based on its physical properties.

Atomic Configuration

Atomic configuration refers to the arrangement of electrons around the nucleus of an atom. It describes how electrons populate different energy levels and sublevels.

• Electrons fill orbitals starting from the lowest energy level to higher ones, following the 'Aufbau principle'.
• Configurations provide insights into the chemical reactivity of elements.
• For example, Neon has the configuration of \(1s^2 2s^2 2p^6\), indicating a full outer shell, which makes it stable and non-reactive.

This stability is a hallmark for noble gases, each having unique electron configurations that avoid unpaired electrons, resulting in their inert nature.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom or ion. For noble gases, this metric is particularly high due to their stable electronic configuration.

• It generally increases across a period and decreases down a group in the periodic table due to the increasing nuclear charge and electron shielding effects.
• Helium, with only one electron shell, has the highest ionization energy among the noble gases.
• High ionization energies reflect the stability of a full valence shell, making it difficult to remove electrons.

This concept emphasizes why noble gases are seldom found in compounds with other elements.

Atomic Size

Atomic size refers to the distance from the nucleus to the outer boundary of the surrounding cloud of electrons.

• Across a period on the periodic table, atomic size decreases due to increased nuclear charge pulling electrons closer.
• Conversely, it increases down a group as additional electron shells are added, making atoms larger.
• Helium has the smallest atomic size among noble gases due to having just one electron shell.

Understanding these trends helps explain why helium is not only the smallest noble gas but also features the highest ionization energy, contributing to its exceptional stability.