Types of SubstanceEdexcel GCSE Chemistry: Revision notes

Elements and compounds can be classified into four structural types: ionic, simple molecular (covalent), giant covalent, or metallic. The structure and bonding of a substance determines its physical properties, including:

• Relative melting and boiling points
• Relative solubility in water
• Ability to conduct electricity

Ionic compounds have high melting and boiling points because of the strong electrostatic forces of attraction between oppositely charged ions in the giant lattice, which require a lot of energy to overcome.

Conductivity depends on state:

• Solid: ions are held in fixed positions in the lattice and cannot move, so they do NOT conduct electricity
• Molten or dissolved in water (aqueous): ions are free to move and carry charge, so they DO conduct electricity

Simple molecular (covalent) substances have low melting and boiling points because only weak intermolecular forces exist between molecules (not the strong covalent bonds within each molecule) — little energy is needed to overcome these weak forces.

They generally have poor electrical conductivity because the molecules have no overall charge and there are no free ions or electrons to carry current.

Graphite and diamond are both giant covalent forms of carbon (allotropes), but have very different structures:

• Diamond: each carbon atom forms four strong covalent bonds in a rigid, tetrahedral 3D lattice — this makes it extremely hard, so it is used in cutting tools
• Graphite: each carbon atom forms three covalent bonds, creating layers of hexagonal rings; the layers are held together by weak forces, so they can slide over each other, making graphite soft and useful as a lubricant. Graphite also has one delocalised electron per carbon atom that can move and carry charge, so it conducts electricity and can be used to make electrodes

Fullerenes (including C60) are cage-like or tube-like molecules of carbon atoms; graphene is a single layer of graphite, one atom thick. Their structures give useful properties: graphene's delocalised electrons make it a good conductor, and both have a very large surface area to volume ratio.

Using poly(ethene) as an example, simple polymers consist of very large molecules made of long chains of carbon atoms, formed by joining together many small monomer units.

Metals are typically shiny solids with high melting points and high density. They are malleable (can be hammered/bent into shape without breaking) because the layers of positive metal ions can slide over each other while still being held together by the delocalised electrons. They are good conductors of electricity because the delocalised electrons are free to move throughout the structure and carry charge.

All representations of structures (dot and cross, ball and stick, 2D/3D diagrams) have limitations — for example, they can exaggerate bond angles, don't show the true scale, or oversimplify the electron arrangement.