Covalent bonds

🔹 1. What is a Covalent Bond?

• A covalent bond is formed when two non-metal atoms share a pair of electrons.

• Each atom contributes one or more electrons to the shared pair.

• The shared electrons create a strong attraction between the atoms.
  💡 Key point: Atoms share electrons to get a full outer shell = stable.

🔹 2. Dot and Cross Diagrams
You’ll need to:

• Draw molecules like H₂, O₂, N₂, Cl₂, H₂O, CO₂, CH₄, NH₃.

• Show only outer shells.

• Use dots for one atom, crosses for the other.
  💥 Higher Tier Tip: Show lone pairs where necessary (like in H₂O and NH₃).

🔹 3. Types of Covalent Molecules
A. Simple Molecular Substances

• E.g. H₂O, CO₂, CH₄, Cl₂

• Low melting/boiling points → weak intermolecular forces

• Don’t conduct electricity

B. Giant Covalent Structures

• E.g. Diamond, Graphite, Silicon Dioxide (SiO₂)

• High melting points

• Insoluble

• Usually don’t conduct (except graphite)

🔹 4. Properties & Explanation
You must explain why they behave the way they do:

🔹 5. Polar and Non-Polar Bonds (Stretch/Higher)

• Non-polar: Atoms share electrons equally (e.g. Cl₂, O₂)

• Polar: Unequal sharing due to electronegativity difference (e.g. HCl, H₂O)
  🧠 You won’t need to calculate electronegativity, but know the idea of partial charges (δ+ and δ−).

🔹 6. Metallic vs Ionic vs Covalent (Comparison)
Know how to compare bonding types:

🔹 7. Advanced Bits to Impress the Examiner (Higher Stretch)

• Describe delocalised electrons in graphite as being free to move between layers.

• Talk about directional covalent bonds in diamond making it hard.

• Explain how intermolecular forces affect boiling points (e.g. bigger molecules = stronger forces = higher b.p.).
  🧠 Practice Tip:
  If your exam question says: