Covalent bonding is not limited to the sharing of a single pair of electrons between two atoms. To achieve a stable noble gas configuration, atoms often form multiple bonds or combine in complex geometries to create large molecules. Furthermore, the physical behavior of these molecules is governed not only by the bonds within them but also by the forces between them. The IGCSE 0654 Extended syllabus requires a detailed description of complex covalent molecules, such as ethene ($C_2H_4$) and methanol ($CH_3OH$), and a rigorous explanation of intermolecular forces.

When atoms share more than one pair of electrons, they form multiple bonds. These bonds are stronger and shorter than single bonds.

Double Bonds (Sharing 2 Pairs)

• Oxygen ($O_2$): Each oxygen atom has six valence electrons. By sharing two pairs, both atoms achieve a full shell of eight.
• Ethene ($C_2H_4$): Contains a carbon-carbon double bond ($C=C$). Each carbon atom also forms two single bonds with hydrogen atoms.

Triple Bonds (Sharing 3 Pairs)

• Nitrogen ($N_2$): Nitrogen atoms have five valence electrons. They share three pairs to achieve stability. This triple bond is exceptionally strong, explaining why nitrogen gas is so unreactive in the atmosphere.

Methanol is a primary alcohol and an example of a molecule containing different types of covalent bonds.

• Carbon Center: Forms three single bonds with hydrogen ($C-H$) and one single bond with oxygen ($C-O$).
• Hydroxyl Group: The oxygen atom is also bonded to a hydrogen atom ($O-H$).
• Dot-and-Cross representation: Shows the overlapping shells of all six atoms, with shared pairs representing each bond.

To explain the physical properties of covalent substances, we must distinguish between two types of attraction:

Intramolecular Forces (Bonds)

These are the strong covalent bonds that hold the atoms together inside a single molecule. These bonds are very difficult to break and are only broken during chemical reactions.

Intermolecular Forces

These are the weak attractive forces that exist between individual molecules.

• Melting and Boiling: When a covalent substance melts or boils, it is these intermolecular forces that are overcome. The strong covalent bonds remain intact; the molecules simply move further apart.
• Low Boiling Points: Because these forces are weak, very little thermal energy is required to separate the molecules, resulting in the characteristic low boiling points of simple molecular substances.

The strength of intermolecular forces increases as the size of the molecule increases.

• Larger Molecules: Have more electrons and a greater surface area, leading to stronger intermolecular attractions.
• Consequence: Larger covalent molecules (like those in the heavier fractions of petroleum) have higher melting and boiling points than smaller ones (like methane).

The diversity of the organic world is a result of the flexible nature of covalent bonding. By sharing multiple pairs of electrons and forming varied molecular geometries, atoms create the complex structures necessary for life and industry. By recognizing the critical role of weak intermolecular forces, we can explain the physical transitions of these molecules, from the volatile gases of the atmosphere to the liquids and solids of the Earth's crust.