Question

What is a molecular solid? What kinds of forces hold molecular solids together?

Short answer

A molecular solid is a solid in which molecules are held together by intermolecular forces such as London dispersion forces, dipole-dipole interactions, and hydrogen bonds.

Step-by-step solution

Define Molecular Solid

A molecular solid is a type of solid consisting of a stable three-dimensional array of molecules held together by noncovalent forces. In these solids, the constituent particles are molecules, which can be the same (as in solid oxygen) or different (as in sucrose).

Identify the Forces in Molecular Solids

Molecular solids are held together by intermolecular forces. These include weaker forces like London dispersion forces, dipole-dipole interactions, and hydrogen bonds. The exact nature of the forces involved depends on the specific molecules that make up the solid.

Key concepts

What is a Molecular Solid?

A molecular solid is a distinctive type of matter, where individual molecules, each with their own set of atoms bonded to each other, compose the structure of the solid itself. These molecules arrange themselves in a highly organized, three-dimensional lattice or network, creating a rigid structure that constitutes the solid form. Unlike ionic or metallic solids, the molecules within a molecular solid retain their identity and do not share electrons across the entire structure.

For instance, dry ice (solid carbon dioxide) showcases a molecular solid structure. Each carbon dioxide molecule remains separate from its neighbors, yet they collectively form the solid we can see and touch. This unique composition of molecular solids gives rise to a varied set of properties, such as lower melting points and softer textures in comparison to other types of solids due to the nature of the forces holding them together.

Noncovalent Forces

Noncovalent forces are the glue that binds molecules together within molecular solids without the need for electron sharing as seen in covalent bonds. These forces are generally weaker than covalent or ionic bonds and are crucial for the integrity and physical properties of the solid. Noncovalent forces can range from weaker London dispersion forces to stronger hydrogen bonds. The importance of these forces cannot be overstated as they are responsible for key biological phenomena, like the three-dimensional structure of proteins and DNA, as well as the behavior of many everyday materials.

The idea that these low-strength interactions can collectively have a significant effect is paramount to understanding molecular solids. When many such interactions are present, they can exercise considerable influence over the physical properties of a substance. This explains why molecular solids are generally softer and have lower melting points.

Intermolecular Forces

Intermolecular forces, a subset of noncovalent forces, are interactions that occur between molecules, holding them close to each other in different phases—solid, liquid, or gas—without the molecules sharing or transferring electrons to form bonds. These interactions are responsible for many physical properties such as boiling point, melting point, and solubility.

In molecular solids, the interplay of these intermolecular forces results in the formation of a structured solid, albeit with a strength and stability that is significantly less than the intramolecular forces within the molecules themselves. Additionally, different types of intermolecular forces can influence the material's physical characteristics, such as volatility and viscosity.

London Dispersion Forces

London dispersion forces are the weakest form of intermolecular attractions, present between all molecules, whether they are polar or nonpolar. These forces originate from the transient dipoles that occur when electrons in an atom or molecule momentarily distribute unevenly, creating an instantaneous dipole that can induce a complementary dipole in a neighboring atom or molecule.

The strengths of London dispersion forces are dependent on the size of the molecule: larger molecules with more electrons have stronger dispersion forces. These forces become particularly significant in nonpolar molecules that lack other types of stronger intermolecular forces. Despite their relative weakness, London dispersion forces play a crucial role in the structural integrity of molecular solids composed of nonpolar molecules.

Dipole-Dipole Interactions and Hydrogen Bonds

Dipole-dipole interactions and hydrogen bonds represent stronger types of intermolecular forces that can determine a molecular solid's physical properties. Dipole-dipole interactions occur between polar molecules where the positive end of one dipole is attracted to the negative end of another. This is typically seen in small polar molecules such as sulfur dioxide (SO₂).

Hydrogen bonds, on the other hand, are a special type of dipole-dipole interaction where hydrogen is involved. They form when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, experiences attraction to a lone pair of electrons on another electronegative atom. These bonds are stronger than standard dipole-dipole interactions and can greatly influence boiling points and melting points. A classic example of hydrogen bonding is water (H₂O), where hydrogen bonds are responsible for many of its unique properties, like high surface tension and the fact that solid water (ice) is less dense than its liquid form.