Problem 79 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following arrangements represents the correct order of electron gain enthalpy (with negative sign) of the given atomic species? (a) \(\mathrm{S}<\mathrm{O}<\mathrm{Cl}<\mathrm{F}\) (b) \(\mathrm{Cl}<\mathrm{F}<\mathrm{S}<\mathrm{O}\) (c) \(\mathrm{F}<\mathrm{Cl}<\mathrm{O}<\mathrm{S}\) (d) \(\mathrm{O}<\mathrm{S}<\mathrm{F}<\mathrm{Cl}\)

Step-by-Step Solution

Answer

The correct order is (d) O < S < F < Cl.

Step 1: Understanding Electron Gain Enthalpy

Electron gain enthalpy is the amount of energy released when an electron is added to a neutral atom in the gas phase, forming a negatively charged ion. A more negative electron gain enthalpy indicates a greater release of energy, signifying a stronger preference for gaining an electron.

Step 2: Analyzing Atomic Sizes and Electron Affinities

Generally, among nonmetals, electron gain enthalpy becomes more negative across a period and less negative down a group due to atomic size and effective nuclear charge. Smaller atoms with higher effective nuclear charge attract additional electrons more strongly, leading to more negative electron gain enthalpies.

Step 3: Comparing Given Atomic Species

We need to analyze the electron gain enthalpies for S (Sulfur), O (Oxygen), Cl (Chlorine), and F (Fluorine). Oxygen and sulfur are in the same group, while fluorine and chlorine are in another group, with chlorine having a more negative electron gain enthalpy than fluorine despite being below it in the group, due to electron-electron repulsion in F.

Step 4: Establishing Correct Order

Considering the trends, chlorine has a more negative electron gain enthalpy than sulfur and oxygen, and fluorine has a less negative enthalpy than chlorine but more than oxygen and sulfur. This results in the order of O < S < F < Cl in increasing negativity.

Key Concepts

Atomic Size

Atomic size refers to the distance from the nucleus of an atom to the outermost electron shell. As you move down a group in the periodic table, atomic size increases because additional electron shells are added. Conversely, atomic size decreases across a period from left to right because the increased nuclear charge pulls electrons closer to the nucleus.

The atomic size is essential when understanding electron gain enthalpy. Larger atoms tend to have less negative (or more positive) electron gain enthalpies due to the electrons being further from the nucleus and experiencing less attraction to the added electron. This makes it more challenging for larger atoms to attract additional electrons compared to smaller atoms.

Effective Nuclear Charge

Effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom. It is determined by the actual nuclear charge, which is the charge of the protons, minus the shielding or repulsion effect caused by other non-core electrons.

As you move across a period, the effective nuclear charge increases, which results in a greater attraction between the nucleus and any added electrons. This is why elements across a period usually have more negative electron gain enthalpies—they can gain new electrons more easily, releasing more energy. This understanding is crucial to predicting the behavior of elements in terms of their electron affinity.

Electron Affinity

Electron affinity is the ability of an atom to accept an additional electron. It is often directly linked with electron gain enthalpy, although they have opposite signs. A high electron affinity means the atom releases more energy when it gains an electron, correlating to a more negative electron gain enthalpy.

In groups where increased atomic sizes are apparent (as you move down), electron affinity generally decreases due to increased distance from the nucleus and lesser effective nuclear charge on outer electrons. Significant exceptions occur due to specific factors like electron-electron repulsions, as seen with fluorine, which despite being the most electronegative element, has a less negative electron gain enthalpy than chlorine due to such repulsions.

Atomic Species Comparison

When comparing atomic species like sulfur, oxygen, chlorine, and fluorine, various trends in electron gain enthalpy can be observed. Chlorine has the most negative electron gain enthalpy, despite being below fluorine in the periodic table. This is due to electron-electron repulsion present in smaller fluorine atoms which slightly reduces its electron affinities. This anomaly is an interesting deviation from the general trend.

Analyzing such trends helps in arranging atomic species in order of increasing negativity of electron gain enthalpy, leading to the observed order: oxygen < sulfur < fluorine < chlorine. This comparison provides insights into the underlying electronic configuration and interactions within atoms, aiding in the systematic understanding of chemical properties.