Simple covalent molecules

Covalent bonds form between non-metal atoms.
Each covalent represents a shared pair of electrons.
Covalent bonds are very strong.

There are two types of covalently bound molecule: simple molecules and giant covalent macromolecules (click here).

Simple covalent molecules

Simple molecular substances consist of molecules in which the atoms are joined by strong covalent bonds.

Examples include the covalently bonded molecules of water, ammonia, hydrogen, hydrogen chloride, methane and oxygen

[Wikimedia commons] Ammonia. Covalent molecule. Nitrogen atom forms a single covalent bond with each of the three hydrogen atoms.

The above Dot and cross simulations are available on BBC Bitesize (click here)

Properties of simple covalent molecules

Substances that consist of simple molecules are gases, liquids or solids that have:

  • relatively low melting points and low boiling points because the molecules are held together by weak intermolecular forces that break down easily
  • do not conduct electricity because the molecules do not have any free electrons or an overall electric charge.

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Difference between intermolecular (between molecules) and intramolecular (within the molecule) forces

Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces).
It is these intermolecular forces that are overcome, not the strong intramolecular covalent bonds, when the substance melts or boils.

The molecule’s intermolecular forces are weak in comparison with the molecule’s intramolecular covalent bonds

BBC Bitesize simulation of weak intermolecular forces between water molecules [click here]

Why does liquid water expand as it freezes?

At atmospheric pressure, when liquid water is cooled, it contracts like one would expect until 4°C.

After that, as it cools, it expands; at its freezing point (0°C) the water has expanded by 9%.

As water cools, there is a greater tendency to form intermolecular hydrogen bonds that are arranged in a ‘spaced apart’ ordered hexagonal crystalline structure. It is this more open structure that causes the ice to be less dense than liquid water, and that is why ice floats on water.

The density of ice is 0.9167 g/cm3 at 0 °C, whereas water has a density of 0.9998 g/cm³ at the same temperature.

[Wikimedia Commons] On average, the crystalline structure of ice (right image) holds the water molecules further apart than how they are held in liquid water (left image).
[Wikimedia commons] Crystalline structure of ice. Grey dashed lines indicate hydrogen bonds.

Revision Questions

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Video showing how water expands as it freezes

The effect of expansion during freezing can be dramatic, and ice expansion is a basic cause of freeze-thaw weathering of rock and flooding of houses when water pipes burst due to the pressure of expanding water in winter sub-zero temperatures.

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