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) \(1 \mathrm{~s}^{2}, 2 \mathrm{~s}^{2}, 2 \mathrm{p}^{3}\) (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) \(1 \mathrm{~s}^{2}, 2 \mathrm{~s}^{2}, 2 \mathrm{p}^{2}\)

Step-by-Step Solution

Answer

The element with the highest electron affinity is fluorine (Option c).

Step 1: Understand Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gaseous phase, forming an anion. The higher the electron affinity, the more energy is released when an electron is added.

Step 2: Analyze the Electronic Configurations

Compare each given electronic configuration. Option (a) is for nitrogen, (b) is oxygen, (c) is fluorine, and (d) is carbon. Understanding these configurations will help determine which element has the highest electron affinity.

Step 3: Assess the Trend of Electron Affinity

Electron affinity generally increases across a period from left to right in the periodic table. This means elements like fluorine have higher electron affinities than elements like nitrogen or carbon which are earlier in the period.

Step 4: Identify the Element with the Highest Electron Affinity

From the previous steps, option (c) \(1s^2, 2s^2, 2p^5\) corresponds to fluorine, which is known to have the highest electron affinity among the given elements.

Key Concepts

Periodic Table Trends

The periodic table is organized in such a way that we can observe various trends as we move across periods (rows) and down groups (columns). These trends include changes in atomic size, ionization energy, and electron affinity. 1. **Across a period:** As you move from left to right across a period, elements show an increase in electron affinity. This is because atoms tend to become more eager to accept an additional electron to achieve a stable configuration, often making them more energy efficient. 2. **Down a group:** Conversely, as you move down a group, electron affinity generally decreases. The additional electron shells added to the atoms make the outer electrons less tightly held as they are further from the nucleus, resulting in less energy release when an additional electron is added. The trend of increasing electron affinity is particularly noticeable across the second period, where fluorine, with its almost complete p-orbital, exhibits one of the highest electron affinities due to its high effective nuclear charge and small atomic radius.

Electron Configuration

Electron configuration refers to the distribution of electrons among the orbital shells and subshells of an atom. Understanding electron configurations is crucial in predicting things like electron affinity and chemical reactivity. Let's break it down: 1. **Basic structure:** Each electron shell is composed of one or more subshells. For example, the first shell has an s-subshell, while the second shell has both s and p-subshells. The given configurations include these subshells. 2. **Notation example:** Take fluorine, which is option (c) with configuration **\(1s^2, 2s^2, 2p^5\)**. This notation denotes the number of electrons in each respective subshell. In total, fluorine has 9 electrons. 3. **Identifying elements:** By identifying which of the given configurations corresponds to each element (like fluorine), we can determine the element's electron configuration and predict their electron affinity easily. Since fluorine is keen on acquiring one more electron for a stable configuration, its electron affinity is notably high.

Chemical Energy Changes

The concept of chemical energy changes is essential when discussing electron affinity. When elements gain or lose electrons, energy is absorbed or released accordingly. This is how it works: 1. **Gaining an electron:** For elements like fluorine, adding an electron results in a release of energy, signifying a negative electron affinity. This process involves a gaseous atom gaining an electron to form a negative ion, releasing energy that reflects the strength of the newly added electron attraction to the nucleus. 2. **Energy comparison:** Typically, elements with high electron affinity, such as those on the far right of a period, release more energy when gaining an electron, as seen with fluorine. In contrast, elements closer to the left of the periodic table tend to have lower or even positive electron affinities because gaining an electron doesn't lead to a significantly more stable electronic configuration. 3. **Chemical insights:** Understanding these energy changes helps in predicting and explaining chemical reactions and the properties of elements. It makes clear why certain elements form specific types of bonds, based on their tendency to gain or lose electrons and the energy changes involved.