- Define and explain Structures and Properties in your own words
- Use key terms such as ion accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Structures and Properties: link giant and molecular structures to melting points and conductivity.
Link giant and molecular structures to melting points and conductivity.
Key ideas
Covalent bonding is electron sharing
Two non-metal atoms share pairs of electrons so that each gains a full outer shell — in methane (CH₄), carbon shares four pairs with four hydrogen atoms. Small molecules like CH₄ and H₂O have weak forces between the molecules, so they melt and boil easily despite the strong bonds within each molecule. Giant covalent structures like diamond are different: every atom is joined by strong covalent bonds, making them extremely hard with very high melting points.
Ionic bonding is electron transfer
When a metal meets a non-metal, the metal loses electrons to form positive ions while the non-metal gains them to form negative ions — for example, sodium loses one electron and chlorine gains one, giving Na⁺ and Cl⁻. The oppositely charged ions attract strongly in all directions, building a giant lattice. This is why ionic compounds have high melting points: melting means breaking those strong attractions.
Key term — ion: An atom or molecule with an electric charge, formed by gaining or losing electrons.
Sodium chloride melts at 801 °C but methane melts at −182 °C. Explain the difference using bonding.
Sodium chloride is ionic: a giant lattice of Na⁺ and Cl⁻ ions held by strong electrostatic attraction in all directions. Melting it means breaking these strong attractions between ions, which needs a great deal of energy, giving a high melting point. Methane is a simple covalent molecule: strong covalent bonds within each CH₄ molecule, but only weak intermolecular forces between molecules. Melting methane only overcomes the weak forces between molecules, needing little energy, giving a very low melting point.
Answer: NaCl's high melting point comes from strong ionic attractions throughout a giant lattice; methane's low melting point is because only weak forces between its small molecules need breaking.
- Saying the covalent bonds break when ice melts Melting breaks the weak forces between molecules, not the strong covalent bonds within them. The H₂O molecules stay intact when ice melts — that is why water is still water.
- Claiming ionic compounds conduct when solid Solid ionic compounds do not conduct because their ions are locked in the lattice and cannot move. They conduct when molten or dissolved, because the ions are then free to carry charge.
Practice
N₂ is a small covalent molecule with only weak intermolecular forces between molecules, so little energy is needed to separate them — hence a gas.
Magnesium loses its 2 outer electrons to form Mg²⁺; oxygen gains 2 electrons to form O²⁻. The Mg²⁺ and O²⁻ ions attract to form magnesium oxide.
Diamond has a rigid 3D network of covalent bonds, making it hard. Graphite has layers with delocalised electrons between them, and these mobile electrons carry charge, so it conducts.
The layers of positive ions can slide over each other without breaking the metallic bond, because the sea of delocalised electrons simply moves with them.
Quick check
Which of these best defines "ion"?
Hydrogen chloride is covalent, yet its solution in water conducts electricity. Explain.
Explain why molten sodium chloride conducts electricity but solid sodium chloride does not.
- Structures and Properties: link giant and molecular structures to melting points and conductivity.
- Covalent bonding is electron sharing: Two non-metal atoms share pairs of electrons so that each gains a full outer shell — in methane (CH₄), carbon shares four pairs with four hydrogen atoms.
- ionic bond: The strong electrostatic attraction between oppositely charged ions.
- Watch out for: saying the covalent bonds break when ice melts