Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following order is correct 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) \(\mathrm{I}_{2}>\mathrm{Br}_{2}>\mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

Option (a) Cl₂ > Br₂ > I₂.

Step 1: Understand the Concept of Bond Energy

Bond energy is the amount of energy required to break one mole of a particular bond in a molecule in the gas phase. It is a measure of bond strength in a chemical bond.

Step 2: Recall Bond Energy Trend in Halogens

For diatomic halogen molecules, bond energies generally decrease as we move down the group in the periodic table: from fluorine (F) to iodine (I). However, chlorine (Cl) is an exception, having a higher bond energy than bromine (Br) due to the smaller atomic size and more effective orbital overlap.

Step 3: Arrange Halogens Based on Bond Energy

Based on the trend and exception, the bond energies for the halogen molecules follow the order: Cl₂ > Br₂ > I₂. Chlorine has the highest bond energy due to its atomic size and effective overlap. Bromine comes next, followed by iodine.

Step 4: Select the Correct Option

Compare the given options to the bond energy order determined in Step 3. The option that matches the order Cl₂ > Br₂ > I₂ is option (a).

Key Concepts

Halogen Molecules

Halogen molecules are an essential class of elements in the periodic table. They belong to Group 17 and include elements like fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Each halogen has seven valence electrons, making them highly reactive as they strive to complete their outer electron shell by gaining one more electron. This reactivity explains why they typically exist as diatomic molecules. The unique properties of halogens arise from their atomic structure, specifically their ability to form covalent bonds easily, resulting in molecules like

• Fluorine molecule: egin{align*} F_2 ewline ext{(g)} ewline ext{ ext{}} ewline Example ewline of ewline halogen ewline molecule} ewline }
• Chlorine: Cl₂
• Bromine: Br₂
• Iodine: I₂

Understanding these molecules' characteristics is crucial for comprehending other chemical interactions, such as reactions and compounds they form.

Diatomic Molecules

Diatomic molecules are composed of two atoms. This term is not limited by the type of chemical bonding involved. Halogens, being highly reactive, often form diatomic molecules as their most stable molecular form. These diatomic molecules include

• Chlorine (Cl₂)
• Fluorine (F₂)
• Other Halogens
• Iodine (I₂, used in water purification and photography)
• Bromine (Br₂, in flame retardants and agricultural practices)
• .

Additionally, oxygen (O₂), nitrogen (N₂), and hydrogen (H₂) molecules are examples, but these aren't halogens. The covalent bond formed between atoms in a diatomic molecule holds them together, maximizing stability. The formation of these molecules highlights an essential aspect of chemical bonding: the quest for lower energy states and stability. By forming bonds, atoms achieve lower energy states, which is pivotal for the occurrence of chemical reactions.

Chemical Bonding

Chemical bonding is an integral concept in chemistry, explaining how atoms combine to form molecules. The energy required to break these bonds is termed bond energy, a direct measure of the bond's strength. In the case of halogens, they predominantly form covalent bonds by sharing electrons. Factors Influencing Bond Strength:

• Atomic size
• Orbital overlap
• Electronegativity

In halogens, for instance, atomic size influences bond energies. Smaller atoms like fluorine have less effective orbital overlap due to repulsion between their tightly bound electrons, leading to lower bond energies compared to chlorine. Chlorine's stronger bond arises because its atomic size enables better orbital overlap than its group peers like bromine or iodine. Thus, understanding chemical bonding involves recognizing how these factors contribute to bond energies and stability in molecules.

Periodic Table Trends

Periodic table trends provide valuable insights into the properties of elements, including both individual atoms and molecules like halogens. The periodic table's arrangement not only shows groups and periods but also indicates trends in properties such as atomic size, electronegativity, and ionization energy. Key Trends:

• Atomic Size: Decreases from left to right across a period and increases down a group
• Electronegativity: Increases across a period and decreases down a group

Understanding these trends is crucial for predicting molecular behavior and bond strength. For halogens, as you go down the group, atomic size increases, orbital overlap becomes less efficient, and as a result, bond energies decrease. However, exceptions, like chlorine having a higher bond energy than bromine, can occur due to such overlaps being particularly effective in achieving stability. Recognizing these patterns helps in predicting elemental behavior not just in isolation but also when elements form compounds or interact during chemical reactions.