Molecular solids | Intermolecular forces and properties | AP Chemistry | Khan Academy

Summary

The video discusses molecular solids, contrasting them with ionic and metallic solids. It explains how nonmetals, like iodine and carbon dioxide, form molecules held together by weak intermolecular forces called London dispersion forces, resulting in relatively low melting points. Solid iodine melts at 113.7°C, while solid carbon dioxide sublimates at -78.5°C. The instructor highlights that the size and polarizability of molecules influence their melting points, demonstrating that larger molecules generally exhibit stronger dispersion forces, thus leading to higher melting points. Overall, the video offers insights into the properties and behaviors of molecular solids.

Takeaways

• 😀 Molecular solids consist of molecules held together by weak intermolecular forces rather than strong covalent bonds.
• 🔗 Covalent bonds occur between atoms in a molecule, creating the individual units that form molecular solids.
• 🌡️ Molecular solids typically have relatively low melting points compared to ionic and metallic solids due to the weak dispersion forces.
• 🧊 Solid iodine (I₂) has a melting point of 113.7 °C, which is relatively low for a solid but higher than that of many other solids.
• ❄️ Dry ice (solid carbon dioxide, CO₂) sublimates directly from solid to gas at -78.5 °C, bypassing the liquid state due to weak intermolecular forces.
• ⚡ Molecular solids, unlike metallic solids, are generally poor conductors of electricity because they lack free-moving charge carriers.
• 🔍 Larger molecules, such as iodine, have stronger dispersion forces due to their increased polarizability, leading to higher melting points compared to smaller molecules like chlorine (Cl₂).
• 🌬️ The melting point of molecular chlorine is -101.5 °C, making it a much colder solid compared to iodine.
• 💡 London dispersion forces arise from temporary dipoles, which can induce dipoles in neighboring molecules, facilitating weak attractions.
• 🌌 Understanding molecular solids helps in studying various real-world applications, including material science, cryogenics, and the behavior of gases at low temperatures.

Q & A

What are molecular solids?

Molecular solids are composed of molecules held together by intermolecular forces, such as dispersion forces, rather than by covalent or ionic bonds.

How do intermolecular forces contribute to the structure of molecular solids?

Intermolecular forces, particularly dispersion forces, hold the molecules together in a solid form. These forces arise from temporary dipoles that induce dipoles in neighboring molecules.

What is the role of covalent bonds in molecular solids?

Covalent bonds hold the atoms within each molecule together, while the molecules themselves are held together by weaker intermolecular forces.

Why do molecular solids have relatively low melting points?

Molecular solids have relatively low melting points because the intermolecular forces that hold them together are weak compared to the strong covalent or ionic bonds found in other types of solids.

What are London dispersion forces?

London dispersion forces are a type of intermolecular force that arises due to temporary dipoles formed when the distribution of electrons around atoms fluctuates.

Can you provide an example of a molecular solid?

Solid iodine (I2) and solid carbon dioxide (CO2, known as dry ice) are examples of molecular solids.

What happens to solid carbon dioxide when heated?

When solid carbon dioxide is heated, it sublimates directly from a solid to a gas without passing through a liquid phase, occurring at a temperature of -78.5 degrees Celsius.

How do the melting points of iodine and chlorine compare?

Iodine has a higher melting point (113.7 degrees Celsius) compared to chlorine (-101.5 degrees Celsius) because iodine has stronger dispersion forces due to its larger, more polarizable atoms.

Why are molecular solids generally poor conductors of electricity?

Molecular solids are poor conductors of electricity because they lack free-moving charged particles, such as electrons, which are present in metallic solids.

What factors determine the strength of dispersion forces in molecular solids?

The strength of dispersion forces is influenced by the size of the molecules and their polarizability; larger molecules with more electrons tend to have stronger dispersion forces.