How does bond strength affect a crystal’s hardness and melting point?

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A rough, translucent yellow crystal of elemental sulfur displayed against a solid black background. — crystal A crystal form of elemental sulfur. Sulfur has an atomic weight of about 32.06 atomic mass units (amu), which means that its molar mass is 32.06 grams.

In crystalline solids, the physical properties of a material are largely determined by the nature and strength of the chemical bonding between its constituent atoms, ions, or ionic groups.1

Impact of Bond Strength

The hardness and melting point of a crystal generally increase in proportion to the strength of the bonds holding the structure together.1

Hardness: Stronger electrical forces require more energy to disrupt the atomic arrangement, which results in greater hardness. For example, the exceptionally strong bonds between carbon atoms in diamond contribute to its extreme hardness.1
Melting Point: A higher melting point indicates that more heat is required to overcome the attractive forces between atoms. A clear comparison is seen between periclase (MgO) and halite (NaCl); because periclase possesses stronger ionic bonds, it melts at 2,800 °C, whereas halite melts at the significantly lower temperature of 801 °C.1

Conversely, as bond strength increases, the coefficient of thermal expansion typically decreases.1