📘 GCSE Chemistry · Notes & worksheet
Bonding and Structure.
Notes
Three types of bonding
| Type | Between | How |
|---|---|---|
| Ionic | metal and non-metal | electrons transferred, forming oppositely charged ions that attract (electrostatic forces) |
| Covalent | non-metals | electrons shared in pairs |
| Metallic | metal atoms | positive ions in a "sea" of delocalised electrons |
Dot-and-cross example: magnesium chloride
Mg loses 2 electrons (to become Mg²⁺) and each of two Cl atoms gains one (to become Cl⁻). Formula MgCl₂. Show outer shells only, with the charge and brackets around each ion.
Properties and explanations
| Substance type | Melting point | Conducts? | Explanation |
|---|---|---|---|
| Giant ionic (NaCl) | high | when molten or dissolved only | strong electrostatic forces between ions need lots of energy; ions free to move only when liquid |
| Simple molecular (H₂O, CO₂) | low | no | weak intermolecular forces overcome, not covalent bonds; no charged particles |
| Giant covalent (diamond, SiO₂) | very high | no (graphite is the exception) | many strong covalent bonds must be broken |
| Metals | high | yes (solid and liquid) | delocalised electrons carry charge; layers of ions can slide, so metals are malleable |
Carbon structures
- Diamond: each carbon bonded to four others. Very hard, very high melting point, does not conduct.
- Graphite: each carbon bonded to three others in layers. Layers slide (weak forces between layers), so it is soft and slippery, and the fourth delocalised electron makes it conduct.
- Graphene: a single layer of graphite. Strong, light, very good conductor.
- Fullerenes: hollow cages or tubes (for example C₆₀, nanotubes). Used in drug delivery and as catalyst supports.
Nanoparticles
1–100 nm. Very large SA:V ratio. If the side of a cube halves, SA:V doubles. Uses: sunscreens, medicine, catalysts. Risks are not fully known.
Polymers
Long chains with strong covalent bonds within chains and weak intermolecular forces between chains, so they are usually solids at room temperature but soften on heating.
Common mistakes
- Saying covalent bonds break when a simple molecular substance melts.
- Saying "ionic compounds have strong bonds so are hard and flexible". They are brittle: a shift in the lattice brings like charges together.
- Forgetting that graphite is a non-metal that conducts.
Exam tips
- For "explain the melting point" questions, say what type of force is overcome and whether it is strong or weak.
Worksheet
Prints the questions only, without answers. Choose “Save as PDF” in the print window to download.
Total: 28 marks.
- Describe, in terms of electrons, how magnesium chloride (MgCl₂) forms from magnesium and chlorine. (4)
- Explain why sodium chloride has a high melting point but conducts electricity only when molten or dissolved. (4)
- Water has a much lower melting point than sodium chloride. Explain why, naming the particles and forces involved. (3)
- Explain why graphite conducts electricity but diamond does not. (3)
- Explain why graphite is soft and can be used as a lubricant. (2)
- Explain why metals can be bent and shaped without breaking, and why they conduct electricity. (4)
- A cube-shaped nanoparticle has side length 10 nm. A second has side length 5 nm.
(a) Calculate the SA:V ratio of each. (3)
(b) State how the ratio changes when side length halves, and give one use of nanoparticles that depends on this. (2) - Explain why poly(ethene) softens when heated but diamond does not. (3)
Show mark scheme and answers (tutor copy)
Try the worksheet first, then check your work here.
- Magnesium loses two electrons from its outer shell (B1) to form Mg²⁺ (B1). Each chlorine atom gains one electron (B1) to form Cl⁻, so two chlorine atoms are needed (B1). The oppositely charged ions attract by electrostatic forces.
- It is a giant lattice of oppositely charged ions held by strong electrostatic forces (B1). Lots of energy is needed to overcome them, so the melting point is high (B1). In a solid the ions are fixed and cannot move (B1). When molten or dissolved the ions are free to move and carry charge (B1).
- Water is a simple molecular substance (B1). Melting overcomes only the weak intermolecular forces between molecules (B1), which need little energy. The strong covalent bonds within each molecule do not break. (B1)
- In graphite each carbon bonds to three others, leaving one delocalised electron (B1) that can move between layers and carry charge (B1). In diamond all four outer electrons are used in covalent bonds, so there are no free electrons (B1).
- Graphite has layers of carbon atoms (B1) held together by weak forces, so the layers slide over each other easily (B1).
- The layers of positive ions can slide over each other (B1) while the sea of delocalised electrons keeps them bonded (B1). The delocalised electrons are free to move through the structure (B1) and carry charge, so metals conduct (B1).
- (a) 10 nm cube: SA = 6 × 100 = 600 nm², V = 1000 nm³, SA:V = 0.6 : 1 (B1, B1). 5 nm cube: SA = 6 × 25 = 150 nm², V = 125 nm³, SA:V = 1.2 : 1 (B1).
(b) The SA:V ratio doubles (B1). Use: catalysts work faster because more surface is exposed; sunscreens; drug delivery. (B1) - Poly(ethene) is made of chains with weak intermolecular forces between them (B1). These weak forces are easily overcome by heating, so the chains can slide (B1). Diamond has a giant covalent structure and all atoms are held by strong covalent bonds that need a huge amount of energy to break. (B1)