Question

Which of the following is a molecular solid? (a) \(\mathrm{I}_{2}\) (b) wax (c) ice (d) all of these

Short answer

(d) All of these

Step-by-step solution

Identify molecular solids

A molecular solid is composed primarily of molecules held together by Van der Waals forces, dipole-dipole interactions, or hydrogen bonds, rather than by covalent or ionic bonds in a lattice structure.

Option (a) 436

Iodine (436) is made up of diatomic molecules held together in a solid by weak van der Waals forces, making it a molecular solid.

Option (b) Wax

Wax is composed of long-chain hydrocarbon molecules. The solid form of wax consists of these molecules held together by weak van der Waals forces, so it is a molecular solid.

Option (c) Ice

Ice is composed of water molecules (04) in which each 0 atom forms hydrogen bonds with neighboring molecules, forming a solid network of molecules. Thus, ice is a molecular solid.

Determine if all options are molecular solids

Since 436, wax, and ice can each be classified as molecular solids based on their intermolecular bonding, the answer is that all of these are molecular solids.

Key concepts

Van der Waals Forces

Van der Waals forces are weak intermolecular forces that occur due to temporary changes in electron density. This leads to temporary dipoles, even in nonpolar molecules.
These forces are often the weakest intermolecular force, but they still play a critical role in holding molecular solids together.

• *London dispersion forces*: These are present in all atoms and molecules due to the random movement of electrons creating fleeting dipoles.
• *Dipole-induced dipole forces*: Occur when a polar molecule induces a dipole in a nonpolar molecule.

The strength of Van der Waals forces can increase with the size and complexity of the molecule, which is why larger molecules like iodine ( \( \mathrm{I}_2 \)) or long-chain hydrocarbons in wax can still form stable molecular solids.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules. These interactions happen because polar molecules have regions of partial positive and partial negative charges.
These regions attract each other, similar to magnets.
This force is stronger than Van der Waals forces but weaker than hydrogen bonds. In molecular solids, these interactions can significantly contribute to the stability of the structure. Some key points include:

• Polar molecules align themselves so that opposite charges are near each other, optimizing the attractive interactions.
• These interactions are affected by molecular shape and the strength of the dipole (i.e., the difference in electronegativity between the atoms).

As an example, in a theoretical molecular solid primarily held by dipole-dipole interactions, the molecules arrange themselves to maximize these attractive forces, though the solids discussed in the exercise rely more heavily on other forces.

Hydrogen Bonds

Hydrogen bonds are particularly strong types of dipole-dipole interactions. These occur when hydrogen is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine.
The electronegativity of these atoms causes hydrogen to have a partial positive charge, which can strongly attract lone pairs on electronegative atoms in nearby molecules.
This bond, while not as strong as covalent or ionic bonds, is much stronger than other forms of intermolecular forces:

• Hydrogen bonds are essential in molecular solids like ice, where each water molecule hydrogen-bonds with four others, creating a highly interconnected and stable crystalline structure.
• These bonds greatly influence the properties of the substances, including boiling and melting points, which are typically higher due to these strong interactions.
• In biological systems, hydrogen bonds are crucial for the structure of DNA and proteins due to their varying strength and flexibility.

In molecular solids like ice, this network of hydrogen bonds makes them stable and gives rise to unique properties, such as expansion upon freezing.