Question

Ionic solids are generally considerably harder than most molecular solids. Explain.

Short answer

Ionic solids are generally harder than molecular solids due to the strong electrostatic forces between the positive and negative ions in their three-dimensional lattice arrangement. In contrast, molecular solids are held together by weaker intermolecular forces, such as van der Waals forces, dipole-dipole interactions, or hydrogen bonds. As a result, ionic solids require more energy to separate the ions compared to the energy needed to separate molecules in molecular solids, leading to a higher hardness in ionic solids.

Step-by-step solution

Bonding in ionic compounds

Ionic compounds are composed of positive ions (cations) and negative ions (anions) that are held together by strong electrostatic forces. These forces exist between all the ions in the solid lattice, as they maintain a regular and organized three-dimensional arrangement. Due to the strong electrostatic attraction between the ions, it requires a large amount of energy to separate them, which makes the ionic compounds hard.

Bonding in molecular compounds

Molecular compounds, on the other hand, consist of individual molecules held together by weaker intermolecular forces such as van der Waals forces, dipole-dipole interactions, or hydrogen bonds. These forces are significantly weaker compared to the electrostatic forces found in ionic compounds. As a result, the amount of energy required to separate the molecules in a molecular solid is relatively small compared to that required for ionic solids.

Comparing hardness

Given the differences in the bonding of both types of solids, ionic solids are generally harder than molecular solids. This is mainly due to the strong electrostatic forces present in ionic solids, which require a large amount of energy to overcome, whereas the weaker intermolecular forces in molecular solids can be more easily broken. This difference in bond strengths ultimately results in the generally higher hardness observed in ionic solids compared to molecular solids.