Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

The electron affinities of \(\mathrm{N}, \mathrm{O}, \mathrm{S}\) and \(\mathrm{Cl}\) are (a) \(\mathrm{O} \approx \mathrm{Cl}<\mathrm{N} \approx \mathrm{S}\) (b) \(\mathrm{O}<\mathrm{S}<\mathrm{Cl}<\mathrm{N}\) (c) \(\mathrm{N}<\mathrm{O}<\mathrm{S}<\mathrm{Cl}\) (d) \(\mathrm{O}<\mathrm{N}<\mathrm{Cl}<\mathrm{S}\)

Step-by-Step Solution

Answer

The correct answer is (c) \( \mathrm{N} < \mathrm{O} < \mathrm{S} < \mathrm{Cl} \).

Step 1: Understanding Electron Affinity

Electron affinity measures the energy change when an electron is added to a neutral atom, forming a negative ion. Generally, across a period from left to right, electron affinity increases, and down a group, it decreases.

Step 2: Analysis of Each Element

Nitrogen (4N5) has a very low electron affinity due to its half-filled 1p5 orbitals being stable. Oxygen (3O5) has a higher affinity since it's one element past nitrogen, but it experiences repulsion in its doubly occupied orbitals. Sulfur (3S5) is below oxygen on the periodic table and thus has a slightly higher affinity due to less repulsion. Chlorine (3Cl5) has the highest electron affinity in its period due to its position in the periodic table, gaining stability by completing its valence shell.

Step 3: Comparing Electron Affinities

From the analysis, 1N5 has the lowest electron affinity since adding an electron disrupts its stable configuration. 1O5, while having higher affinity than 1N5, is less than 1S5 due to smaller size causing more repulsion. 1S5 is more favorable due to less repulsion compared to 1O5. 1Cl5 has the highest electron affinity of the four because it's close to achieving a full stable shell.

Step 4: Choosing the Correct Sequence

Given the explanations, the correct order of increasing electron affinity is 1N5, 1O5, 1S5, 1Cl5, making option (c) 1N < O < S < Cl5 the correct answer.

Key Concepts

Periodic Table Trends

The periodic table organizes elements in a way that helps us predict their chemical properties and behaviors. One of the key trends affected by position on the periodic table is electron affinity. As you move from left to right across a period, the electron affinity generally increases. This is because the atoms become smaller, and the additional electron is added closer to the nucleus, which desires electrons more strongly.

• Left to Right Across a Period: Electron affinity increases due to stronger nuclear charge pulling electrons closer.
• Top to Bottom in a Group: Electron affinity typically decreases since larger atomic size weakens the pull on added electrons.'

Understanding these trends aids in predicting and comparing the electron affinities of elements like Nitrogen, Oxygen, Sulfur, and Chlorine.

Energy Change in Atom

Electron affinity is essentially about energy change. When an electron is added to a neutral atom, this process can either release or require energy. An atom that releases energy upon gaining an electron has a positive electron affinity, indicating a more stable state. Conversely, requiring energy means the atom resists gaining an extra electron, signaling lower affinity.

• Energy release upon gaining an electron means a higher electron affinity.
• Elements release more energy if the additional electron results in a more stable electron configuration.

The energy change involved in electron affinity can reveal much about the stability and reactivity of each element. Chlorine, for example, releases a significant amount of energy as it prefers a complete outer shell, resulting in its high electron affinity.

Halogen Stability

Halogens, like Chlorine, are incredibly reactive due to their electron configurations, which lack just one electron to achieve full stable shells. These elements reside in Group 17 of the periodic table and are known for having relatively high electron affinities.

• Halogens have high electron affinities because they are eager to complete their outer electron shells.
• This eagerness makes halogens highly reactive and willing to accept electrons from other elements.

Chlorine’s high electron affinity stems from its ability to become more stable by gaining an electron, making it one of the most reactive elements in its group.

Orbital Configurations

The arrangement of electrons in an atom’s orbitals significantly affects its electron affinity. Each element has a unique electron configuration that influences how it interacts with additional electrons. Half-filled or fully filled orbitals are particularly stable and resist changes in electron configuration.

• Nitrogen: With half-filled p orbitals, its low electron affinity is due to the stability of this configuration.
• Oxygen: Has unpaired electrons but experiences electron-electron repulsion, lowering affinity compared to equivalent orbitals like Sulfur.
• Sulfur: Experiences less repulsion than Oxygen, resulting in slightly higher electron affinity.

Each element's specific orbital configuration influences its electron affinity by determining how desirable it is for additional electrons to enter their valence shells.