Why do molecular compounds have low melting points and low boiling points relative to ionic substances?
Most molecular compounds are composed of discrete molecules whereas ionic compounds are composed of a three dimensional array of ions. To melt an ionic compound the attractive forces of the crystal must be broken. There are some exceptions to this rule; e.g., diamond is very hard but it is composed of carbon in a network of covalent bonds.
Molecular compounds have low melting and boiling points relative to ionic substances due to the difference in their bonding nature. To understand this, let's break it down:
1. Bonding in molecular compounds: Molecular compounds are composed of atoms that share electrons, creating covalent bonds. These bonds are formed through the sharing of valence electrons between atoms. As a result, the attractive forces between molecules, known as intermolecular forces, are relatively weak compared to the strong electrostatic forces in ionic compounds.
2. Intermolecular forces: There are three types of intermolecular forces that affect molecular compounds: van der Waals forces, dipole-dipole interactions, and hydrogen bonding. These forces arise from temporary shifts in electron density and differences in electronegativity. However, they are much weaker than the strong electrostatic forces between oppositely charged ions in ionic compounds.
3. Melting and boiling points: Melting and boiling points are measures of the energy required to break the intermolecular forces and transition a substance from a solid to a liquid or a liquid to a gas. Since the intermolecular forces in molecular compounds are weak compared to the strong ionic bonds in ionic compounds, less energy is needed to overcome these forces. As a result, molecular compounds have low melting and boiling points.
In summary, the weak intermolecular forces in molecular compounds lead to low melting and boiling points compared to the strong electrostatic forces in ionic compounds. To predict and compare the melting and boiling points of different compounds, it is essential to understand their bonding nature and the strength of the intermolecular forces involved.