Understanding First Ionisation Energy

First Ionisation Energy is defined as the minimum energy required to remove the outermost electron from a neutral gaseous atom in its ground state. It's typically measured in kilojoules per mole ($kJ \ mol^{-1}$).

Several factors influence ionization energy, including:

• Nuclear Charge: Higher nuclear charge leads to stronger attraction for electrons, increasing ionization energy.
• Atomic Radius: Larger atoms have their outermost electrons further from the nucleus, requiring less energy to remove them, thus decreasing ionization energy.
• Electron Shielding: Inner-shell electrons shield the outer electrons from the full nuclear charge, reducing the effective nuclear charge experienced by outer electrons and lowering ionization energy.
• Electron Configuration Stability: Atoms with stable electron configurations, such as a completely filled valence shell ($ns^2np^6$) or a half-filled subshell ($np^3$), have higher ionization energies due to their extra stability.

Generally, ionization energy increases across a period (from left to right) and decreases down a group.

Analysing Electronic Configurations (List-I)

We need to understand the characteristics of each electronic configuration provided in List-I:

• (A) $ns^2$: This configuration represents elements in Group 2 of the periodic table (alkaline earth metals). They possess a stable, completely filled s-subshell.
• (B) $ns^2np^1$: This configuration corresponds to elements in Group 13. They have a filled s-subshell and one electron in the p-subshell.
• (C) $ns^2np^3$: This configuration is characteristic of Group 15 elements. They have a stable, half-filled p-subshell ($np^3$), which confers extra stability.
• (D) $ns^2np^6$: This configuration represents Group 18 elements (noble gases). They have a completely filled valence shell, representing the highest level of stability.

Matching Configurations with Ionisation Energies (List-II)

Now, let's relate these configurations to the approximate first ionization energies given in List-II, considering the trends and stability factors:

• Highest Ionisation Energy: The configuration $ns^2np^6$ (noble gases) is the most stable. Therefore, it requires the most energy to remove an electron. Among the given values, 2100 $kJ \ mol^{-1}$ (I) is the highest. Thus, (D) should correspond to (I).
• Next Highest Ionisation Energy: The configuration $ns^2np^3$ (Group 15) has a stable half-filled p-subshell. This makes its ionization energy relatively high, generally higher than configurations like $ns^2$ or $ns^2np^1$ in the same period, but lower than noble gases. The value 1400 $kJ \ mol^{-1}$ (II) fits this description. Thus, (C) should correspond to (II).
• Comparison between $ns^2$ and $ns^2np^1$: According to general periodic trends, ionization energy increases across a period. However, there's a notable exception between Group 2 ($ns^2$) and Group 13 ($ns^2np^1$). Elements in Group 13 generally have a *lower* first ionization energy than Group 2 elements in the same period. This is because the electron being removed in Group 13 is in a slightly higher energy p-orbital, and the effective nuclear charge doesn't increase enough to compensate fully. Therefore, we expect $IE(ns^2np^1) < IE(ns^2)$. Looking at the remaining values, 800 $kJ \ mol^{-1}$ (III) and 900 $kJ \ mol^{-1}$ (IV), the pairing (B) - (III) and (A) - (IV) fits this expectation ($800 < 900$).

Step-by-Step Matching Explanation

Let's summarize the matching based on the analysis:

• Configuration (D) $ns^2np^6$: Represents noble gases, which have a fully filled, highly stable valence shell. Requires the highest ionization energy. Matches with (I) 2100 $kJ \ mol^{-1}$.
• Configuration (C) $ns^2np^3$: Represents Group 15 elements with a stable half-filled p-subshell. Requires a high ionization energy, but less than noble gases. Matches with (II) 1400 $kJ \ mol^{-1}$.
• Configuration (A) $ns^2$: Represents Group 2 elements. Has a stable filled s-subshell. Ionization energy is generally moderate. Matches with (IV) 900 $kJ \ mol^{-1}$.
• Configuration (B) $ns^2np^1$: Represents Group 13 elements. Due to the slightly higher energy of the p-electron and incomplete shielding, its ionization energy is lower than Group 2 elements. Matches with (III) 800 $kJ \ mol^{-1}$.

This leads to the following correct matching:

• (A) - (IV)
• (B) - (III)
• (C) - (II)
• (D) - (I)

Summary Table

The matching between electronic configurations and their approximate first ionization energies is summarized below: