The shapes of molecules (groups of atoms bonded together) and the way…

The idea

The arrangement of atoms in molecules and giant structures determines how a substance behaves. In simple molecules, small groups of atoms are held together by strong covalent bonds. The specific 3D shape of these molecules affects how they interact, though weak intermolecular forces mean they have low melting points. Giant structures contain huge numbers of atoms or ions in regular lattices. Diamond and graphite are giant covalent structures of carbon, but their different atomic arrangements mean diamond is hard, while graphite is soft and conducts electricity. Similarly, the regular arrangement of positive ions and delocalised electrons in giant metallic structures allows metals to be bent and shaped.

What earns the marks

• Simple molecular substances consist of small groups of atoms joined by strong covalent bonds, but weak intermolecular forces give them low melting and boiling points.
• The specific 3D shape of a simple molecule influences how it interacts with other molecules and determines its overall physical properties.
• Giant covalent structures, such as diamond, have atoms arranged in a rigid 3D lattice held by strong covalent bonds, making them very hard with high melting points.
• Graphite has carbon atoms arranged in hexagonal layers with weak forces between them, allowing the layers to slide, which makes it soft and slippery.
• Metals form giant metallic lattices where layers of positive ions can slide over each other, explaining why metals are malleable and ductile.
• Ionic compounds form giant ionic lattices of oppositely charged ions, which have high melting points due to strong electrostatic forces of attraction in all directions.

Common mistakes and how to put them right

• Thinking that intermolecular forces are covalent bonds.

  Put it right by stating that weak forces act between molecules, while strong covalent bonds act within each molecule.

• Believing graphite is soft because its covalent bonds are weak.

  Put it right by explaining that the covalent bonds within the layers are strong, but the weak intermolecular forces between the layers allow them to slide.

• Confusing the structures of different giant lattices.

  Put it right by ensuring metals are described as giant metallic lattices (positive ions and delocalised electrons) and not giant ionic lattices (oppositely charged ions).

Exam tips

• When asked to explain a physical property, identify the type of structure first (e.g., simple molecular, giant covalent), then link the specific bonding or arrangement to the property.
• Use the phrase 'weak intermolecular forces' when explaining the low melting points of simple molecules, rather than saying 'weak bonds'.