Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

The electronic configurations of some elements are given below. The element with highest electron affinity is (a) \(\mathrm{ls}^{2}, 2 \mathrm{~s}^{2}, 2 \mathrm{p}^{3}\) (b) \(1 \mathrm{~s}^{2}, 2 \mathrm{~s}^{2}, 2 \mathrm{p}^{4}\) (c) \(1 \mathrm{~s}^{2}, 2 \mathrm{~s}^{2}, 2 \mathrm{p}^{5}\) (d) \(\mathrm{ls}^{2}, 2 \mathrm{~s}^{2}, 2 \mathrm{p}^{2}\)

Step-by-Step Solution

Answer

Element (c) with configuration \(1s^{2}, 2s^{2}, 2p^{5}\), Fluorine, has the highest electron affinity.

Step 1: Understand Electron Affinity

Electron affinity is the energy change when an atom in the gas phase accepts an electron, forming a negative ion. This property typically becomes more negative across a period due to higher nuclear charge making it easier to add electrons.

Step 2: Determine Electronic Configurations

Review the given electronic configurations: - (a) \( 1s^{2}, 2s^{2}, 2p^{3} \) corresponds to Nitrogen (N).- (b) \( 1s^{2}, 2s^{2}, 2p^{4} \) corresponds to Oxygen (O).- (c) \( 1s^{2}, 2s^{2}, 2p^{5} \) corresponds to Fluorine (F).- (d) \( 1s^{2}, 2s^{2}, 2p^{2} \) corresponds to Carbon (C).

Step 3: Compare Electron Affinities

- Nitrogen (a) has a half-filled p-shell, leading to a lower electron affinity because it is relatively stable. - Oxygen (b) will have a higher electron affinity than nitrogen but less than fluorine. - Fluorine (c), being the most electronegative element, will have the highest electron affinity. - Carbon (d) has an even fewer number of p-electrons, which results in even lower affinity than oxygen.

Step 4: Identify the Element with Highest Electron Affinity

Fluorine, from option (c), has the highest electron affinity because it is more effective at stabilizing added electrons due to its high electronegativity and small atomic radius.

Key Concepts

Electronic Configuration

When we talk about the electronic configuration of an atom, we're discussing the arrangement of electrons in various shells and subshells around the nucleus of an atom. Each element has a unique electronic configuration that determines its chemical properties. Understanding this arrangement helps predict how an atom will interact with others.

• The electronic configurations are usually written using numbers to denote the energy level and letters to show the subshell (like s, p, d, f) followed by a superscript number indicating the number of electrons in that subshell.

For example, the configuration \(1s^2, 2s^2, 2p^3\) for nitrogen means there are 2 electrons in the 1s subshell, 2 in the 2s, and 3 in the 2p subshell. This information is crucial for understanding trends like electron affinity because it shows how electrons fill the available orbital spaces, influencing an element's reactivity and properties.

Periodic Trends

Periodic trends are patterns seen within elements on the periodic table. One such trend is electron affinity, which generally increases across a period and decreases down a group.
This occurs because, as you move across a period from left to right, atoms have more protons in their nuclei. This increased "nuclear charge" pulls electrons closer, making it easier for the atom to attract additional electrons.

• Across a period: Electronegativity and electron affinity increase.
• Down a group: Both properties generally decrease due to more electron shells and increased distance from the nucleus.

Understanding these periodic trends helps predict which atoms are more likely to gain electrons, a crucial factor in determining electron affinity, as well as in bonding and chemical reactivity.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold on to electrons within a chemical bond. Unlike electron affinity, which deals with a single atom in the gas phase, electronegativity concerns atoms in molecules.
Fluorine is the most electronegative element, explaining its high electron affinity. The small atomic radius of fluorine allows for strong attractions between the nucleus and added electrons.

• High electronegativity leads to higher electron affinity because atoms like fluorine can stabilize additional electrons effectively.
• Electronegativity is a key determinant of how atoms interact in compounds.

Understanding electronegativity helps in predicting molecule behavior, and explaining why elements like fluorine so readily gain electrons, making them extremely reactive.