Question

Covalent bonding occurs in both molecular and covalent-network solids. Why do these two kinds of solids differ so greatly in their hardness and melting points?

Short answer

The difference in hardness and melting points between molecular and covalent-network solids is due to the forces holding the atoms together within these structures. Molecular solids are held together by weak intermolecular forces, such as van der Waals forces or hydrogen bonding, leading to lower melting points and softer structures. On the other hand, covalent-network solids have a continuous network of strong covalent bonds throughout the entire solid, resulting in higher melting points and harder structures.

Step-by-step solution

1. Understanding molecular solids

Molecular solids are formed when individual molecules are held together by weak intermolecular forces such as van der Waals forces or hydrogen bonding. These forces are relatively weak compared to the covalent bonds holding the atoms together within the individual molecules. Therefore, molecular solids tend to have lower melting points and are relatively soft compared to covalent-network solids.

2. Understanding covalent-network solids

Covalent-network solids, on the other hand, are formed by a continuous network of covalent bonds between atoms throughout the entire solid. This results in a strong and rigid structure with a high degree of connectivity between the atoms. Due to the strong covalent bonds, covalent-network solids tend to have much higher melting points and are much harder than molecular solids.

3. Comparison of molecular and covalent-network solids

The key difference between molecular and covalent-network solids lies in the forces holding the atoms together in these structures. While molecular solids rely on weaker intermolecular forces, covalent-network solids have strong covalent bonds throughout the structure. This leads to pronounced differences in their hardness and melting points, with covalent-network solids being generally harder and having higher melting points than molecular solids.

4. Examples of molecular and covalent-network solids

Examples of molecular solids include water ice, carbon dioxide, and sugar. In each of these cases, the weak intermolecular forces lead to relatively low melting points and relatively soft structures. In contrast, examples of covalent-network solids include diamond, silicon dioxide (quartz), and silicon carbide. These materials are characterized by their high melting points and extreme hardness due to the strong covalent bonds in their extended networks. In conclusion, the main reason why molecular and covalent-network solids differ greatly in their hardness and melting points is the nature of the forces holding the atoms together within the structures. Molecular solids rely on weaker intermolecular forces, while covalent-network solids are held together by strong covalent bonds throughout the material.

Key concepts

Covalent Bonding

Covalent bonding is a type of chemical bond where two atoms share a pair of electrons. This sharing allows each atom to attain the electron configuration of a noble gas, leading to a stable molecular structure. In covalent bonds, the electron pair is shared in a way that the positive nuclei of the bonded atoms are attracted to the same electron pair, creating a strong and stable bond.

When we consider solids, covalent bonding can occur in two primary forms: within molecules, which are the building blocks of molecular solids, and as a continuous network in covalent-network solids. The strength and arrangement of these covalent bonds are pivotal when determining the physical properties of the solid, such as hardness and melting point. For instance, diamond is an archetypal example of a covalent-network solid in which each carbon atom is tetrahedrally bonded to four other carbon atoms, forming an incredibly hard structure that's very difficult to break apart.

In comparison, in molecular solids like ice, the water molecules are held together by weaker forces, and the covalent bonds only exist within the discrete molecules, not between them. This makes molecular solids generally much softer and less stable at higher temperatures than covalent-network solids.

Intermolecular Forces

Intermolecular forces are the relatively weak forces that exist between molecules or the discreet units in a solid or liquid. Unlike covalent or ionic bonds, which hold atoms together within molecules or compounds, intermolecular forces hold these molecules or units together in a bulk material. There are several types of intermolecular forces, with the most commonly discussed being van der Waals forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding.

The strength of these intermolecular forces greatly affects the physical properties of a substance. For example, a molecular solid with strong hydrogen bonds will have higher melting and boiling points than one with only London dispersion forces. Since intermolecular forces are much weaker than covalent bonds, molecular solids, which are held together by intermolecular forces, are easier to melt or break apart compared to covalent-network solids, which are bonded by a continuous network of strong covalent bonds.

The melting process involves overcoming these intermolecular forces to transition from a solid to a liquid state, hence why molecular solids with weaker intermolecular forces melt at lower temperatures. As such, ice, composed of water molecules held together by hydrogen bonds, will melt at 0°C, whereas CO₂ solid, or dry ice, which is held together by weaker dispersion forces, sublimates at -78°C, even without turning into a liquid.

Melting Points

The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point, the kinetic energy of the particles overcomes the forces holding them in their fixed positions and the solid starts to become fluid. The melting point is a crucial physical property that is indicative of the strength of the forces at play within the solid's structure.

For molecular solids, which are held together by intermolecular forces, the melting points are generally lower. This is because the energy required to overcome the relatively weak intermolecular forces is significantly less than that needed for breaking covalent bonds. On the other hand, covalent-network solids have significantly higher melting points because they are composed of atoms held together in a vast network of covalent bonds, which require a much higher energy input to break apart.

This concept explains the diverse melting points observed in different solids. For instance, sugar, a molecular solid, melts at about 186°C, whereas diamond, a covalent-network solid, doesn't melt until a temperature of about 3550°C is reached under normal atmospheric pressure. Understanding the correlation between bond types, intermolecular forces, and melting points is crucial for predicting and explaining the thermal properties of different substances, and why materials behave differently under heat.