Difference Between Molecular Solid and Covalent Network Solid

The key difference between molecular solid and covalent network solid is that molecular solid forms due to the action of Van der Waal forces whereas covalent network solid forms due to the action of covalent chemical bonds.

We can categorize solid compounds in different ways – depending on the structure, composition, bonding, properties, applications, etc. Molecular solids, ionic solids, metallic solids, covalent network solids are such different types of solids.

CONTENTS

1. Overview and Key Difference
2. What is Molecular Solid 
3. What is Covalent Network Solid
4. Side by Side Comparison – Molecular Solid vs Covalent Network Solid in Tabular Form
5. Summary

What is Molecular Solid?

A molecular solid is a solid compound containing molecules held together via Van der Waal forces. There are no ionic or covalent bonds between these molecules. The forces between these molecules are cohesive attraction forces. There are different types of Van der Waal forces that can cause the formation of a molecular solid, i.e. dipole-dipole interactions, pi-pi interactions, hydrogen bonding, London forces, etc.

However, these Van der Waal forces are weaker compared to ionic and covalent chemical bonds. Therefore, molecular solids typically have relatively low melting and boiling points. Furthermore, these solids tend to dissolve in organic solvents. These molecular solids have a low density and are non-conductive as well; thus, these are soft electrical insulators.

Moreover, when considering the different allotropes of a chemical element, all allotropes sometimes exist as molecular solids, but most of the times, some allotropes are molecular solids while other allotropes of the same chemical element are not molecular solids. For instance, there are different allotropic forms of phosphorus; we name them as red, white and black phosphorous. Among them, white phosphorous is a molecular solid, but red phosphorous exists as chain structures.

Furthermore, molecular solids are either ductile or brittle depending on the nature of crystal faces of the solid. Both these ductile and brittle forms can undergo elastic deformation as well.

What is a Covalent Network Solid?

Covalent network solids are solid compounds containing atoms bonded to each other via covalent chemical bonds. These solids have a number of repeating atoms linked to each other via covalent bonds. The chemical bonding can cause the formation of a network of atoms, which leads to the formation of a network solid. Therefore, we can consider a covalent network solid as a type of macromolecule.

Furthermore, these solids can occur in two ways; as crystalline solids or amorphous solids. A suitable example for a network solid is diamond with covalently bonded carbon atoms, which forms a strong 3D structure. Usually, covalent network solids have relatively high melting and boiling points. Generally, these solids are insoluble in any type of solvent because it is very difficult to break down the bonds between atoms. Moreover, these solids are very hard and have a low electrical conductivity at its liquid phase. The electrical conductivity at the solid phase may vary according to the composition.

What is the Difference Between Molecular Solid and Covalent Network Solid?

Molecular solids and covalent network solids are two types of solid compounds. The key difference between molecular solid and covalent network solid is that molecular solid forms due to the action of Van der Waal forces whereas covalent network solid forms due to the action of covalent chemical bonds. When considering their properties, molecular solids are relatively soft material, while covalent network solids are very hard.

Moreover, molecular solids have relatively low melting points, whereas covalent network solids have very high melting points. Furthermore, molecular solids are electrical insulators, while covalent network solids have low electrical conductivity at the liquid state and the electrical conductivity at solid phase may vary according to the composition. Water ice is a good example for molecular solids, while diamond is the best example of a covalent network solid.

Below infographic summarizes the difference between molecular solid and covalent network solid.

Summary – Molecular Solid vs Covalent Network Solid

Molecular solids and covalent network solids are two types of solid compounds. The key difference between molecular solid and covalent network solid is that molecular solid forms due to the action of Van der Waal forces whereas covalent network solid forms due to the action of covalent chemical bonds.

Reference:

1. Helmenstine, Anne Marie. “Molecular Solids: Definition and Examples.” ThoughtCo, Dec. 3, 2018, Available here.
2. Helmenstine, Anne Marie. “Network Solid Definition in Chemistry.” ThoughtCo, Jul. 8, 2019, Available here.

Image Courtesy:

1. “Acetic acid hydrogen bonding” By Cryst eng 340 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Carbon dioxide and caffeine” By Cryst eng 340 – Own work (CC BY-SA 4.0) via Commons Wikimedia