Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

Properties that are directly or indirectly related to the electronic configuration of the elements and show a regular gradation when we move from left to right in a period or form top to bottom in a group are called periodic properties. Some of these properties are ionization energy, electron affinity and electronegativity Decreasing order of electron affinity is (a) \(\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}>\mathrm{S}>\mathrm{Si}\) (b) \(\mathrm{Si}>\mathrm{S}>\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}\) (c) \(\mathrm{Cl}>\mathrm{F}>\mathrm{Br}>\mathrm{I}>\mathrm{S}>\mathrm{Si}\) (d) \(\mathrm{Cl}>\mathrm{F}>\mathrm{Br}>\mathrm{S}>\mathrm{Si}>\mathrm{I}\)

Step-by-Step Solution

Answer

The correct order is option (c): Cl > F > Br > I > S > Si.

Step 1: Understanding electron affinity

Electron affinity refers to the amount of energy released when an electron is added to a neutral atom in the gaseous state to form a negative ion. It indicates how strongly an atom attracts electrons.

Step 2: Trends in electron affinity

In the periodic table, electron affinity generally increases from left to right across a period and decreases down a group. However, the trend is not as consistent because some exceptions may exist due to electron configurations.

Step 3: Comparing halogens

Among the given elements, halogens (Group 17) such as Chlorine (Cl), Fluorine (F), Bromine (Br), and Iodine (I) have high electron affinities, with Chlorine having slightly higher electron affinity than Fluorine due to interelectronic repulsions in Fluorine.

Step 4: Position of sulfur and silicon

Sulfur (S) generally has a higher electron affinity compared to Silicon (Si), as Silicon belongs to Group 14 and electron affinity increases across a period. However, Sulfur is generally below halogens in electron affinity.

Step 5: Ordering from highest to lowest electron affinity

Based on the electron affinity trends and comparisons, the order from highest to lowest electron affinity is: Cl > F > Br > I > S > Si.

Key Concepts

Electron Affinity

Electron affinity measures the amount of energy released when an electron is added to a neutral atom. This energy change is often exothermic, meaning energy is released. Due to this property, electron affinity helps us understand how much an atom "wants" an extra electron. The more energy released, the stronger the attraction for an additional electron.

• When an atom gains an electron, it becomes negatively charged, forming an anion.
• Atoms with high electron affinity values are typically more reactive.
• For instance, in the periodic table, chlorine is known for having one of the highest electron affinities.

While electron affinity typically increases across periods and decreases down groups, some exceptions arise due to atomic structural intricacies. For example, fluorine, despite being above chlorine, has lower electron affinity due to electron-electron repulsions.

Periodic Table Trends

In the periodic table, trends for various properties, including electron affinity, ionization energy, and electronegativity, provide a systematic way to understand elements. These trends help predict behavior and reactivity.

• Electron affinity increases across a period from left to right due to increasing nuclear charge, making atoms more eager to gain electrons.
• Conversely, electron affinity generally decreases as you move down a group since atomic size increases, making the added electron feel less attraction.
• These trends are fundamental in predicting how elements will interact and bond with each other.

Halogens

Halogens, located in Group 17 of the periodic table, are known for their high reactivity and high electron affinity values. This is because these elements are very close to achieving a full outermost electron shell, which is a stable electronic configuration.

• Common halogens include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).
• Chlorine has a slightly higher electron affinity than fluorine, despite fluorine's position higher in the group, due to interelectronic repulsion factors.
• The high reactivity of halogens makes them prone to forming negative ions, especially with metals to form salts.

Ionization Energy

Ionization energy refers to the energy required to remove an electron from an atom, forming a cation. It's a crucial property for understanding an element's reactivity towards forming positive ions.

• Higher ionization energy means an atom holds onto its electrons more tightly, making it less reactive in losing electrons.
• Ionization energy increases across a period due to increasing nuclear charge, which strongly holds electrons closer.
• It decreases down a group as atoms get larger and the outer electrons become less tightly bound to the nucleus.

This trend of ionization energy is opposite to that of electron affinity and plays a role in the general reactivity and chemical behavior of elements.

Electronegativity

Electronegativity is the tendency of an atom to attract a bonding pair of electrons. It's an essential concept for determining how electrons are distributed in molecules.

• Atoms with high electronegativity values, like fluorine, are very good at attracting electrons in a bond.
• Electronegativity increases from left to right across a period, as elements strive for a more stable electron configuration.
• It decreases as you move down a group because increased atomic size results in weaker attraction on shared electrons.

This property plays a key role in the type of bond formed between atoms, distinguishing between nonpolar covalent, polar covalent, and ionic bonds.