Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

The van der Waals forces in halogen decrease in the order (a) \(\mathrm{I}_{2}>\mathrm{Br}_{2}>\mathrm{Cl}_{2}>\mathrm{F}_{2}\) (b) \(\mathrm{F}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\) (c) \(\mathrm{I}_{2}>\mathrm{F}_{2}>\mathrm{Br}_{2}>\mathrm{Cl}_{2}^{2}\) (d) \(\mathrm{Cl}_{2}>\mathrm{I}_{2}>\mathrm{Br}_{2}^{2}>\mathrm{F}_{2}\)

Step-by-Step Solution

Answer

Option (a) I_{2} > Br_{2} > Cl_{2} > F_{2} is the correct order.

Step 1: Understanding van der Waals Forces

Van der Waals forces are intermolecular forces that are weaker than covalent or ionic bonds. These forces become stronger as the size and number of electrons of the atoms involved increase.

Step 2: Analyzing Halogens' Atomic Properties

Halogens are Group 17 elements in the periodic table. Moving down the group, the size of the atoms and the number of electrons they have both increase. Hence, Iodine atoms ( I_{2} ) have the most electrons and the largest size, resulting in the strongest van der Waals forces among halogen diatomics.

Step 3: Determining Order Based on Theory

Based on the increase in size and number of electrons, the van der Waals forces should decrease from Iodine to Fluorine, in the order: I_{2} > Br_{2} > Cl_{2} > F_{2} . This is because Iodine is the largest and has the most electrons, whereas Fluorine is the smallest and has the fewest electrons.

Step 4: Identifying the Correct Option

The correct order from the previous step matches with option (a). Hence, the van der Waals forces in halogens decrease in the order: I_{2} > Br_{2} > Cl_{2} > F_{2}.

Key Concepts

Understanding Halogens

Halogens are fascinating elements that belong to Group 17 in the periodic table. This group includes Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). Each halogen is just one electron short of a full outer shell, making them highly reactive as they seek to achieve a stable electron configuration.
Fluorine is the most reactive of the group, often forming compounds with nearly every element. As you move down the group from Fluorine to Iodine, the reactivity of halogens decreases even though the atomic size and number of electrons increase.

• Halogens have distinctive colors in their gaseous state: Fluorine is pale yellow, Chlorine is greenish-yellow, Bromine is reddish-brown, and Iodine is violet.
• They exist as diatomic molecules, meaning they pair up with another atom of the same element (e.g., F₂, Cl₂).

Periodic Table's Role

The periodic table is an organized chart categorizing all known elements based on their atomic number and properties. It reveals trends in the behavior of elements, including the halogens.
One of the key trends observed in the periodic table is the change in atomic size and reactivity. Within a group, as atomic numbers increase, atomic size increases due to the addition of electron shells.

• For halogens, the periodic table helps predict behaviors such as reactivity, bonding patterns, and physical states at room temperature.
• For example, Fluorine and Chlorine are gases, Bromine is a liquid, and Iodine is a solid at room temperature, all attributable to their placement and atomic structure seen in the periodic table.

Exploring Intermolecular Forces

Intermolecular forces are interactions that occur between molecules, playing a crucial role in determining the physical properties of substances. Van der Waals forces are a type of intermolecular forces, weaker than covalent or ionic bonds.
These forces arise from temporary dipoles that occur when electrons in a molecule incidentally cluster on one side, making it slightly charged. In halogens, larger atoms like Iodine have more electrons that can shift to create stronger temporary dipoles, resulting in stronger van der Waals forces.

• Although weak, van der Waals forces increase with the size and number of electrons in the interacting atoms or molecules.
• As a result, as you move from Fluorine to Iodine down the halogen group, these forces increase, making I₂ have the strongest van der Waals forces amongst the halogens.

The Effect of Atomic Size

Atomic size, or atomic radius, impacts many properties of elements, none more than their van der Waals forces. In halogens, as atomic size increases from Fluorine to Iodine, so does the strength of these forces.
The larger an atom, the more electron shells it has, which increases its radius. Iodine, being the largest of the common halogens, fits this description perfectly with the result being its stronger van der Waals forces compared to smaller halogens like Fluorine.

• Atomic size not only affects intermolecular forces but also influences melting and boiling points. Larger atomic size generally leads to higher melting and boiling points.
• Iodine's larger atomic size and high van der Waals forces mean it is a solid, while Fluorine's smaller size means it's a gas under standard conditions.