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Covalent bonding forms substances with two types of structures:

• molecular covalent structures. Simple molecules held together by weak forces of attraction. Examples include iodine, methane and carbon dioxide.
• giant covalent structures. Many atoms joined together by strong covalent bonds. Examples include diamond and graphite.

Molecular covalent structures

The atoms in molecular covalent molecules are held together by strong covalent bonds. Although these bonds are strong, there are only weak forces of attraction between molecules. These weak attractive forces are calledvan der Waals' forces and can be broken with little energy.

Physical properties of molecular covalent structures

• low melting and boiling points. Little energy is needed to break the weak van der Waals' forces between the molecules, so covalent molecular structures have low melting and boiling points.
• do not conduct electricity. Covalent molecular structures do not conduct electricity because the molecules are neutral and there are no charged particles (no ions or electrons) to move and carry charge.
• insoluble in water. Most covalent compounds are insoluble in water. Hydrogen chloride and ammonia are exceptions, because they react with water.