Structure, properties and uses
Simple molecular substances consist of molecules in which the atoms are joined by strong covalent bonds. However, the molecules are held together by weak forces so these substances have low melting and boiling points. They do not conduct electricity.
Giant covalent structures contain many atoms joined together by covalent bonds to form a giant lattice. They have high melting and boiling points. Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.
Covalent bonds [covalent bond: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] form between non-metal atoms. Each bond consists of a shared pair of electrons [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ], and is very strong. Covalently bonded substances fall into two main types:
These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ] of carbon bonded with two atoms of oxygen.
Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.
The animation shows how the weak intermolecular forces between water molecules break down during boiling or melting: