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) \(\mathrm{Cl}_{2}>\mathrm{I}_{2}>\mathrm{Br}_{2}>\mathrm{F}_{2}\)

Step-by-Step Solution

Answer

(a) I_2 > Br_2 > Cl_2 > F_2 .

Step 1: Understanding van der Waals Forces

Van der Waals forces are weak intermolecular forces that occur between atoms and molecules due to temporary fluctuations in electron density. In halogen molecules such as those mentioned, larger molecules with more electrons typically have stronger van der Waals forces. This is because larger molecules are more polarizable, leading to stronger dispersion forces.

Step 2: Identifying Molecular Size and Electrons

Halogens appear in the periodic table in group 17 and are ordered according to their atomic number as follows: Fluorine ( F_2 ), Chlorine ( Cl_2 ), Bromine ( Br_2 ), and Iodine ( I_2 ). Thus, the size and number of electrons increase from F_2 to I_2 . As such, I_2 , having the most electrons and being the largest, experiences the strongest van der Waals forces, followed by Br_2 , Cl_2 , and finally F_2 .

Step 3: Ordering Halogens by Van der Waals Forces

Based on the understanding of van der Waals forces, the correct order from strongest to weakest is: I_2 > Br_2 > Cl_2 > F_2 . This means option (a) is the correct order of decreasing van der Waals forces among the given halogen molecules.

Key Concepts

Halogens

Halogens are a group of elements found in group 17 of the periodic table. They include fluorine, chlorine, bromine, iodine, and astatine. These elements are known for their high reactivity, especially fluorine and chlorine. Halogen atoms tend to gain an electron during chemical reactions to form negative ions. This electron gain leads to the formation of stable, filled outer electron shells.

Halogens exist as diatomic molecules in their elemental form, meaning they pair up as two atoms, like \(F_2\), \(Cl_2\), \(Br_2\), and \(I_2\). This pairing enables them to stabilize by sharing electrons through a covalent bond. Due to their high electronegativity, halogens possess strong tendencies to attract and gain electrons. This property is a major player in their chemical behavior and interactions.

• Fluorine is the most reactive, often participating in vigorous reactions.
• Chlorine is widely used for disinfection and bleaching.
• Bromine is less reactive than chlorine but still very active and commonly used in flame retardants.
• Iodine is the least reactive of this group but crucial for biological processes like thyroid function.

Understanding the properties of halogens helps to explain their physical states and reactivity as they progress from gases to solids down the group.

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion which act between neighboring particles, like atoms or molecules. They are crucial in determining the physical properties of substances, such as boiling and melting points. Among the different types, van der Waals forces are particularly relevant for halogens.

Van der Waals forces consist of dispersion forces, dipole-dipole interactions, and hydrogen bonding, but in nonpolar molecules like halogens, dispersion forces dominate. These forces arise due to temporary fluctuations in electron density creating instantaneous dipoles. These dipoles induce further dipoles in neighboring molecules, leading to an overall attractive force.

• Dispersion forces become stronger with an increase in molecular size.
• These forces are generally weakest among smaller and lighter molecules.
• For halogens, as the atomic mass increases, so does the strength of the van der Waals forces, explaining why \(I_2\) has the strongest forces among them.

Understanding these interactions is vital for grasping how molecules behave and interact, playing a huge role in the structure and properties of substances.

Molecular Size and Polarity

Molecular size and polarity are key factors in determining the strength of intermolecular forces. As molecular size increases, so does the number of electrons, which enhances the polarizability of the molecule. This increase in polarizability leads to more substantial van der Waals forces. This principle explains the trend observed in halogens.

With halogens, moving down the group from \(F_2\) to \(I_2\), the molecular size increases. As a result, the van der Waals forces intensify due to greater dispersion forces. Unlike some other kinds of molecules, halogens in their diatomic form do not have a permanent dipole moment because they are homonuclear diatomics, meaning both atoms in the molecule are identical, making the molecule nonpolar.

• This lack of polarity negates any dipole-dipole interactions.
• In nonpolar molecules, larger size directly leads to stronger forces despite the lack of polarity.
• As such, \(I_2\) experiences the most potent van der Waals forces due to its large size and high electron count.

By understanding the link between molecular size, polarizability, and intermolecular forces, we can better predict and rationalize the properties and behaviors of substances, especially when it comes to halogens.