Problem 10 - Chemistry The Science in Context

Question

At room temperature, bromine \(\left(\mathrm{Br}_{2}\right)\) is a corrosive red liquid, whereas iodine \(\left(\mathrm{I}_{2}\right)\) is a volatile violet solid. The differences point to different strengths of intermolecular forces between these halogens, with those for \(\mathrm{I}_{2}\) being stronger. What kind of intermolecular force is responsible for these differences?

Step-by-Step Solution

Answer

Answer: The intermolecular force responsible for the differences between bromine (Br2) and iodine (I2) is London dispersion forces. The larger size and the higher number of electrons in iodine cause its London dispersion forces to be stronger, resulting in a less volatile, more solid substance at room temperature compared to bromine.

Step 1: Recognize the nature of the molecules

Br2 and I2 are both nonpolar diatomic molecules in their elemental form and are covalently bonded within the molecule.

Step 2: Eliminate irrelevant forces

Since both Br2 and I2 are nonpolar molecules, we can rule out any intermolecular forces that require polar characteristics, such as ionic bonding, hydrogen bonding, and dipole-dipole interactions. This leaves us with London Dispersion Forces.

Step 3: Identify the force responsible for strength differences

London Dispersion Forces are the weakest type of intermolecular force, arising due to the transient induced dipoles created by the movement of electrons. However, the strength of these forces increases with the size and the number of electrons in the atoms. Since iodine is larger and has more electrons than bromine, its London Dispersion Forces are stronger, making it less volatile and more solid at room temperature compared to bromine.

Step 4: Conclusion

The intermolecular force responsible for the differences between bromine (Br2) and iodine (I2) is London dispersion forces. The larger size and the higher number of electrons in iodine cause its London dispersion forces to be stronger, resulting in a less volatile, more solid substance at room temperature compared to bromine.

Key Concepts

London Dispersion Forces

London dispersion forces play a crucial role in understanding how and why molecules interact. These forces are the weakest type of intermolecular attraction and are a subset of van der Waals forces. They arise from the temporary shifts in electron density within nonpolar molecules.

Even though a molecule is nonpolar, the constant movement of its electrons can create a momentary dipole. This temporary dipole can then induce a dipole in a neighboring molecule, leading to a weak attraction between them.

Key features of London dispersion forces include:

• They occur in all molecules, whether polar or nonpolar, as long as the molecules have electrons.
• Their strength increases with more significant electron clouds or larger molar mass. The bigger and heavier the molecule, the stronger the dispersion forces.
• Though weak, they are crucial in determining the physical properties of substances, especially nonpolar ones like bromine ( B_2 ) and iodine ( I_2 ).

Due to these forces, the characteristics such as boiling and melting points of substances are affected. For instance, iodine, being a larger molecule with more electrons, has stronger London dispersion forces compared to bromine, thus being solid at room temperature.

Nonpolar Molecules

Nonpolar molecules are made up of atoms that share electrons equally due to comparable electronegativity or symmetrical arrangement. Bromine ( Br_2 ) and iodine ( I_2 ) are perfect examples of nonpolar diatomic molecules. They consist of two identical atoms that pull equally on the shared electrons, balancing out any potential uneven charge distribution.

Characteristics of nonpolar molecules include:

• No permanent dipole: These molecules lack a permanent separation of charge, which means they don't have positive and negative ends.
• Symmetrical structure: This symmetry often results in uniform charge distribution, further confirming their nonpolarity.
• Weak intermolecular forces: Nonpolar molecules primarily interact through London dispersion forces, which are weaker than the interactions found between polar molecules.

Understanding nonpolar molecules helps explain why compounds like bromine and iodine do not exhibit forces like hydrogen bonding or dipole-dipole interactions, leading to their unique physical states.

Halogens

Halogens are a group in the periodic table known for their reactivity and distinct colors. This group includes fluorine, chlorine, bromine, iodine, and astatine. Each halogen exists as diatomic molecules ( Br_2 , I_2 , etc.) in their natural states.

These elements are essential for various applications, ranging from industry to healthcare. However, when considering halogens like bromine and iodine, it's critical to consider their physical properties shaped by intermolecular forces.

Key characteristics of halogens:

• They have seven electrons in their outer shell, which makes them highly reactive, seeking one more electron to attain stability.
• They can form various compounds, both ionic and covalent, depending on the reacting counterparts.
• In their diatomic states, they engage mainly through London dispersion forces, particularly significant given their electronegativity and size.

Despite their weak intermolecular interactions, the overall size of these molecules results in noticeable differences in physical properties, such as the volatility of bromine versus the solid state of iodine at room temperature. Understanding these subtleties helps explain the behavior and usage of halogens in different contexts.