Relating Bonding to Physical Properties
This topic explains how the four main types of chemical structure (ionic, simple molecular, giant covalent, metallic) arise from bonding and how these structures directly determine a substance's physical properties, especially its melting point and electrical conductivity.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Ionic compounds form giant lattices of oppositely charged ions. The strong electrostatic attraction requires lots of energy to overcome, resulting in high melting points. They conduct electricity only when molten or dissolved, as ions must be mobile to carry charge.
- Simple molecular substances consist of discrete molecules with weak forces between them (intermolecular forces). Little energy is needed to overcome these weak forces, leading to low melting and boiling points. They never conduct electricity as they lack free charge carriers.
- Giant covalent structures are giant lattices of atoms joined by strong covalent bonds. Breaking these bonds requires immense energy, giving them very high melting points. They are typically electrical insulators, with graphite being a key exception due to its delocalised electrons.
- Metals consist of a giant lattice of positive ions in a 'sea' of delocalised electrons. The strong attraction between ions and electrons results in high melting points. The mobile delocalised electrons mean metals always conduct electricity, both as solids and liquids.
Definitions
- Delocalised electrons
- Outer shell electrons that are not associated with a single atom or covalent bond, and are free to move throughout the entire structure. They are responsible for electrical conductivity in metals and graphite.
- Intermolecular forces
- The weak forces of attraction that exist *between* simple molecules. These are overcome when a substance melts or boils, not the strong covalent bonds *within* the molecules.
- Giant Lattice
- A continuous, repeating three-dimensional arrangement of particles (ions or atoms) held together by strong forces. Found in ionic, metallic, and giant covalent structures.
Worked example
The properties of four unknown substances, P, Q, R, and S, are listed in the table. Identify the type of structure for each substance. | Substance | Melting Point / °C | Electrical Conductivity (Solid) | Electrical Conductivity (Liquid) | |---|---|---|---| | P | -114 | Insulator | Insulator | | Q | 1538 | Conductor | Conductor | | R | 801 | Insulator | Conductor | | S | 2230 | Insulator | Insulator |
- 1
Analyse Substance P:
It has a very low melting point (-114 °C) and is always an insulator.
This combination is characteristic of a simple molecular structure with weak intermolecular forces and no free charge carriers.
- 2
Analyse Substance Q:
It has a high melting point (1538 °C) and conducts electricity in both solid and liquid states.
The ability to conduct as a solid is unique to metals, due to delocalised electrons.
This is a metallic structure.
- 3
Analyse Substance R:
It has a high melting point (801 °C), but only conducts electricity when liquid.
This 'switch' in conductivity is the key property of an ionic compound, where ions are fixed in the solid lattice but mobile in the liquid state.
This is an ionic lattice.
- 4
Analyse Substance S:
It has a very high melting point (2230 °C) and is always an insulator.
The very high melting point suggests a giant structure, and the lack of conductivity rules out metallic bonding.
This is a giant covalent structure (like silicon dioxide).
Answer: P: Simple molecular, Q: Metallic, R: Ionic, S: Giant covalent
Common mistakes
- ×Confusing giant covalent with simple molecular structures. Low melting points always indicate simple molecules where only weak intermolecular forces are broken, not the strong covalent bonds present in a giant covalent lattice.
- ×Forgetting the two-state condition for ionic conductivity. A substance with a high melting point that conducts electricity only when molten or aqueous is ionic. If it conducts as a solid, it must be a metal.
- ×Mixing up intramolecular and intermolecular forces. The low boiling points of molecular substances are due to weak intermolecular forces, not the strong covalent bonds within the molecules.
- ×Assuming all giant covalent structures are insulators. Graphite is a common exception that you must know; it conducts electricity due to its layers of delocalised electrons.
No-calculator tips
- ✓Use conductivity as your first sorting tool. If it conducts as a solid, it's a metal. If it only conducts as a liquid, it's ionic. If it never conducts, it's molecular or giant covalent.
- ✓Use melting point as the second sorting tool. A very low melting point (e.g., below 100 °C) almost certainly means it's a simple molecular substance. Very high melting points (over 1000 °C) indicate a giant structure (ionic, metallic, or giant covalent).