Question

Iodine is a solid at room temperature \(\left(\mathrm{mp}=113.5^{\circ} \mathrm{C}\right)\) while bromine is a liquid \(\left(\mathrm{mp}=-7^{\circ} \mathrm{C}\right)\). Explain this difference in terms of intermolecular forces.

Short answer

Iodine has stronger dispersion forces than bromine due to its larger molecular size, making it solid at room temperature.

Step-by-step solution

Understanding Melting Points

The melting point (mp) is the temperature at which a substance changes from solid to liquid. The higher the melting point, the stronger the intermolecular forces holding the molecules together in the solid state.

Identifying Intermolecular Forces

Iodine (I₂) and bromine (Br₂) are nonpolar molecules that experience dispersion forces, also known as London dispersion forces. These are the weakest type of intermolecular forces, but they become stronger with increasing molecular size.

Comparing Molecular Sizes

Iodine molecules are larger than bromine molecules. The bigger molecular size of iodine leads to stronger dispersion forces compared to bromine, resulting in a higher melting point.

Relating Intermolecular Forces to States of Matter

Since iodine has stronger intermolecular forces due to its larger size, it remains a solid at room temperature. In contrast, the weaker intermolecular forces in bromine result in it being a liquid at the same temperature.

Key concepts

Melting Point

Melting point is a fundamental concept in understanding the physical state of a substance at a given temperature. It is defined as the temperature at which a solid turns into a liquid.
This transition occurs when the thermal energy provided to the material overcomes the intermolecular forces holding the molecules in a structured, lattice-like configuration in the solid state.

• A higher melting point indicates stronger intermolecular forces, which means more energy is required to disrupt the solid structure.
• A lower melting point suggests weaker intermolecular forces, requiring less energy to transition into a liquid.

Iodine, with a melting point of 113.5°C, remains solid at room temperature, whereas bromine, which melts at -7°C, is a liquid under the same conditions. This difference highlights the significant role intermolecular forces play in determining the physical states of these substances.

London Dispersion Forces

London dispersion forces, also known as Van der Waals forces, are a type of intermolecular force present in all atoms and molecules. These forces are particularly prominent in nonpolar substances such as diatomic iodine (I₂) and bromine (Br₂).
They arise due to the temporary fluctuations in electron distribution within molecules, leading to temporary dipoles that induce dipoles in neighboring molecules.

• Despite being the weakest form of intermolecular interaction, they can significantly influence physical properties such as melting points and boiling points, especially in larger molecules.
• The strength of London dispersion forces increases with greater electron cloud size, resulting in stronger attractions as molecular size increases.

For iodine and bromine, these dispersion forces explain why iodine, with stronger dispersion forces due to its larger size, has a higher melting point than bromine.

Molecular Size Comparison

The size of a molecule greatly affects its intermolecular forces. In the case of iodine and bromine, the molecular size is a key factor in determining the strength of the dispersion forces at play.

• Larger molecules have more electrons, which can create more substantial temporary dipoles, leading to stronger London dispersion forces.
• Iodine molecules are considerably larger than bromine molecules, contributing to the stronger forces between iodine molecules.

As a result, the stronger intermolecular forces in iodine increase its melting point compared to bromine, making it a solid at room temperature, while bromine, with weaker forces due to its smaller size, remains in liquid form. This comparison illustrates how molecular size directly impacts the physical properties and behavior of substances.