Simple covalent substance

Properties of simple covalent substances.

Also called simple molecular structures.

The vast majority of covalent elements and compounds have this type of structure.

Exceptions include silicon, 2 allotropes of the element carbon (graphite and diamond) and the compound silica (SiO₂).

Consist of separate ‘small’ molecules.
There are intermolecular forces of attraction between the molecules.
These attractions may, or may not, result in intermolecular bonds forming.
There are several types of intermolecular force. However, all arise from an unequal spread or sharing of electrons within the molecule. This gives rise to regions with small/partial charges – denoted with a small delta symbol (δ).
Intermolecular forces are attractions between a δ⁺ region in one molecule and a δ^– region in another molecule.

Intermolecular forces are generally weak, requiring relatively little energy to overcome. Most simple molecular substances are gases or volatile liquids at room temperature.

Question 1.

Going down Group 7 the melting points and boiling clearly increase. The first two are gases, bromine is a liquid and iodine is a solid. Explain this trend.
Descending the group, the molecules have more electrons and a greater surface area.
As a result, the intermolecular forces increase in strength and require more energy to overcome.

Question 2.

Why do simple molecular substances not conduct electricity?

Electricity requires either freely moving ions or delocalised electrons in order to flow. These substances consist solely of molecules and molecules cannot carry charge.

Metals have delocalised electrons that are free flowing. Ionic salts, when molten or in aqueous solution, have free moving ions.

Question 3.

Simple molecular solids (e.g. iodine) are brittle. Explain why.

Iodine molecules have a relatively large surface area making the intermolecular forces stronger. However, intermolecular forces are much weaker than covalent and ionic bonds. It takes little energy to overcome the attractions between iodine molecules, making it brittle.

The image of iodine above was taken from wikipedia.

Click here to see what simple molecular substances look like at the molecular level.

Link to old presentation here.