Problem 105 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is the correct order for the bond energies of halogen molecules? (a) \(\mathrm{Cl}_{2}>\mathrm{Br}_{2}>\mathrm{I}_{2}\) (b) \(\mathrm{Br}_{2}>\mathrm{Cl}_{2}>\mathrm{I}_{2}\) (c) \(\mathrm{I}_{2}>\mathrm{Cl}_{2}>\mathrm{Br}_{2}\) (d) \(\mathrm{I}_{2}>\mathrm{Br}_{2}>\mathrm{Cl}_{2}^{2}\)

Step-by-Step Solution

Answer

The correct order of bond energies is (a) \\(\mathrm{Cl}_{2} > \mathrm{Br}_{2} > \mathrm{I}_{2}\\).

Step 1: Understanding Bond Energies

Bond energy is the measure of bond strength in a chemical bond. The higher the bond energy, the stronger the bond is. For diatomic halogen molecules, bond energies typically decrease as we move down the group in the periodic table because the atoms become larger and the bonds become weaker.

Step 2: Consideration of Halogen Molecules

We are considering the bond energies of the molecules \(\mathrm{Cl}_{2}\), \(\mathrm{Br}_{2}\), and \(\mathrm{I}_{2}\). As we move down the group from chlorine to iodine, the atomic size increases which generally leads to a decrease in bond energy.

Step 3: Comparing Bond Energies

For halogen molecules, the trend in bond energies is: \(\mathrm{Cl}_{2}\) has the highest bond energy, followed by \(\mathrm{Br}_{2}\), and then \(\mathrm{I}_{2}\) with the lowest bond energy. This is due to larger atomic sizes and longer bond lengths as you move down the group, which reduce bond strength.

Step 4: Identifying the Correct Order

Based on typical bond energy values, \(\mathrm{Cl}_{2} > \mathrm{Br}_{2} > \mathrm{I}_{2}\) is the correct order, since chlorine molecules have the strongest bonds among the given halogens due to their smaller atomic size and shorter bond length.

Key Concepts

Halogen Molecules

Halogen molecules, like those formed by the elements chlorine (Cl), bromine (Br), and iodine (I), are highly important in chemistry.
These elements belong to group 17 of the periodic table and are known for forming diatomic molecules, meaning each molecule consists of two atoms.
This is why we see the notations \(\mathrm{Cl}_2\), \(\mathrm{Br}_2\), and \(\mathrm{I}_2\).

Halogen molecules are noted for their reactivity, which is due to their electron configuration. They all need one more electron to complete their valence shell, leading them to readily form bonds.
As you move down the group in the periodic table from chlorine to iodine, several properties change, such as their physical state (chlorine is a gas, bromine is a liquid, and iodine is a solid at room temperature).
This change in state is a result of increasing molecular weight and size, which affects how these molecules interact with each other. Understanding the nature of halogen molecules is key to grasping how their bond energies vary.

Periodic Table Trends

The periodic table is an essential tool in chemistry and provides insight into the properties of elements and their compounds.
One significant trend observable in the periodic table is how the size of atoms changes as you move down a group.
For halogens, this means that iodine atoms are larger than bromine atoms, which in turn are larger than chlorine atoms.

This increase in atomic size is due to the addition of more electron shells as you move down the group.
Larger atoms tend to have longer bond lengths and, therefore, weaker chemical bonds.
These factors directly influence the bond energies of diatomic molecules formed by halogen elements.
Understanding these periodic trends helps us predict and rationalize the behavior of elements like chlorine, bromine, and iodine, particularly in terms of their bonding characteristics.

Chemical Bond Strength

Chemical bond strength is a key concept in understanding how molecules function and react with each other.
Bond strength is typically measured by bond energy, which quantifies the amount of energy required to break a bond in a molecule.
In the context of halogen molecules, \(\mathrm{Cl}_2 > \mathrm{Br}_2 > \mathrm{I}_2\) represents the trend in bond energy as you move down the periodic table.

This trend can be explained by the increasing size of the atoms: larger atoms have more widespread electron clouds, which results in weaker attraction between the two bonded nuclei.
This leads to longer bond lengths and reduced bond strength.
The smaller chlorine atoms can form stronger bonds due to their closer proximity, resulting in higher bond energy.
Recognizing the relationship between atomic size, bond length, and bond strength is critical for understanding why halogen molecules have the bond energies they do.