Question

What is the difference between a covalent molecular solid and a covalent network solid? Do their physical properties differ? Explain your answer.

Short answer

The difference between a covalent molecular solid and a covalent network solid is based on the forces binding their atoms. Covalent molecular solids are held together by non-covalent forces like van der Waals forces and hydrogen bonds, while covalent network solids consist of a continuous network of covalent bonds. These bonding differences result in distinct physical properties: covalent molecular solids generally have lower melting and boiling points, less rigid structures, and poor conductivity. In contrast, covalent network solids have higher melting and boiling points, more rigid structures, better thermal conductivity, but usually poor electrical conductivity.

Step-by-step solution

Definition of Covalent Molecular Solid

A covalent molecular solid is a solid composed of discrete molecules held together by non-covalent forces, such as van der Waals forces and hydrogen bonds. Examples include solid water (ice) and solid carbon dioxide (dry ice).

Definition of Covalent Network Solid

A covalent network solid is a solid in which the atoms are connected by a continuous network of covalent bonds. This results in a three-dimensional lattice structure. Examples of covalent network solids include diamond and quartz (silicon dioxide).

Comparison of Physical Properties

Covalent molecular solids and covalent network solids have different physical properties, mainly because of the differences in the forces holding them together. 1. Covalent molecular solids typically have lower melting and boiling points compared to covalent network solids, as the non-covalent intermolecular forces are weaker than the covalent bonds present in covalent network solids. 2. Covalent molecular solids are usually softer and less rigid compared to covalent network solids, as the intermolecular forces in the former can be easily disrupted under pressure, while the latter's covalent bonds require more energy to break. 3. Covalent molecular solids are generally poor conductors of heat and electricity, as they lack a continuous network of charged particles required for the efficient transfer of energy or charge. Covalent network solids can show better thermal conductivity, such as in the case of diamond, which has an excellent thermal conductivity due to its strong covalent bonds and lattice structure. However, covalent network solids are generally non-conductive of electricity as they lack free electrons or charge carriers.

Conclusion

The difference between a covalent molecular solid and a covalent network solid lies in the type of forces binding their atoms together. Covalent molecular solids are held together by non-covalent forces such as van der Waals forces and hydrogen bonds, whereas covalent network solids are composed of a continuous network of covalent bonds. These differences in bonding result in distinct physical properties, with covalent molecular solids generally exhibiting lower melting and boiling points, less rigid structures, and poor conductivity, while covalent network solids exhibit higher melting and boiling points, more rigid structures, and better thermal conductivity but usually poor electrical conductivity.

Key concepts

Covalent Molecular Solids

Covalent molecular solids consist of molecules bound together principally by non-covalent forces. These forces, like van der Waals forces or hydrogen bonds, are significantly weaker than covalent bonds themselves.
Examples of covalent molecular solids include ice (solid water) and dry ice (solid carbon dioxide). This type of solid typically has low melting and boiling points, because the intermolecular forces are relatively easy to break.
Moreover, covalent molecular solids are generally soft, as their structures are not rigidly held together. They do not conduct electricity because they lack free-moving charge carriers.

Covalent Network Solids

Covalent network solids are composed of atoms interconnected by a vast network of covalent bonds, forming a continuous, three-dimensional structure. This structural pattern is responsible for the distinct properties of these solids.
Typical examples include diamond and quartz. With strong covalent bonds holding the structure, these solids have very high melting and boiling points. Their hardness and rigidity are attributed to the strength of the covalent bonds that make them tough to break.
Due to the absence of free electrons or charge carriers, covalent network solids are usually poor conductors of electricity. However, in some cases like diamond, their excellent lattice structure provides remarkable thermal conductivity.

Physical Properties of Solids

The physical properties of covalent solids differ based on whether they are molecular or network in nature.
Covalent molecular solids tend to be soft, with gentle intermolecular forces allowing deformation and low thermal stability. They also do not conduct electricity due to the absence of mobile charges.
Covalent network solids, on the other hand, are characterized by their exceptional hardness and high melting and boiling points. This results from their tightly bound, consistent lattice structures. While these solids typically lack electrical conductivity, they can display excellent thermal conductivity thanks to their orderly atom arrangement.

• Softness and flexibility in covalent molecular solids due to weaker forces.
• Rigidity and hardness in covalent network solids due to strong covalent bonds.
• Differences in thermal and electrical conductivity based on structural composition.

Melting and Boiling Points

Melting and boiling points are crucial indicators of a solid's stability and structure. In covalent molecular solids, weaker intermolecular attractions mean that less energy is needed to separate the molecules, resulting in lower melting and boiling points.
Covalent network solids demand much more energy to disrupt their extensive network of strong covalent bonds. Consequently, they boast significantly higher melting and boiling points compared to covalent molecular solids.
The difference in bond strength directly influences how these materials behave when subjected to heat, explaining why covalent network solids remain stable at higher temperatures.

Thermal and Electrical Conductivity

Thermal and electrical conductivity showcase how efficiently a material can transmit heat and electricity, respectively. Covalent molecular solids display poor conductivity in both areas. The lack of freely moving charges and weak intermolecular forces inhibit the efficient transfer of heat and electricity.
In contrast, some covalent network solids like diamond demonstrate excellent thermal conductivity. This is due to their orderly structure and strong covalent bonds which facilitate efficient heat transfer. However, they are generally poor electrical conductors because they lack free electrons or ions.
The varying ability of these solids to conduct heat and electricity stems from the nature of their atomic bonds and structures, making them unique for different applications.