Why ionic compounds melt so high and conduct only when molten or dissolved, then sharing electrons in covalent molecules.
What you will learn
- Explain the melting points and conduction of ionic compounds
- Describe a covalent bond as a shared pair of electrons
- Count shared pairs and bonding electrons in simple molecules
A giant lattice of ions
In a solid ionic compound, huge numbers of positive and negative ions sit in a regular three-dimensional pattern called a giant ionic lattice.
Every ion is attracted to the oppositely charged ions around it by strong electrostatic forces acting in all directions.
That explains the properties:
- High melting and boiling points: melting means overcoming a vast number of strong attractions, which takes a lot of energy.
- No conduction as a solid: the ions are locked in place.
- Conducts when molten or dissolved in water: the ions are free to move and carry charge.
It is the ions that move, not electrons. Writing "electrons move" in an ionic answer loses the mark. (Delocalised electrons, free to move through a whole structure rather than staying with one atom, are found in graphite and metals: lessons 4 and 5.)
Compare a substance made of small molecules, such as sugar or wax: it melts at a far lower temperature, because only the weak forces between the molecules (intermolecular forces) are overcome, not strong bonds. Lesson 3 explains these forces.
To test which solids are ionic, use their conduction. Set up a low-voltage power supply, a lamp (or an ammeter) and two carbon electrodes in series. Touch the electrodes onto each dry solid, then dissolve the solid in water and dip the electrodes into the solution. An ionic compound (copper sulfate, sodium chloride) conducts only in solution; a covalent one (sugar, glucose) never conducts. Keep the voltage low, wear eye protection, and rinse the electrodes between tests so one result does not affect the next.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- A giant lattice of ions
- Try it: When does it conduct?
- Sharing instead of transferring
- Counting shared pairs
10 practice questions, marked as you go; about 22 minutes in all.
Key terms
Giant ionic lattice, Share, Covalent bond
In this unit: Bonding, structure and the properties of matter
Ionic, covalent and metallic bonding, the structures they make, and how structure explains melting points, conduction and hardness, with nanoparticles for separate Chemistry.