The shape of the lesson
By the end of the lesson, learners can
- The four lattices: Rotate each lattice in the simulator: sodium chloride, copper, diamond and iodine.
- Carbon, four ways: Switch to the carbon allotropes.
- Ice, iodine and alloys: Finish with the metals and alloys simulator: push a layer of pure metal, then add larger or smaller atoms and push again.
- Giant ionic: ions, ionic bonds.
- Giant metallic: cations and delocalised electrons.
- Giant molecular: atoms, covalent bonds everywhere.
- Simple molecular: molecules, weak intermolecular forces.
One picture every idea returns to
A crystalline solid has a regular three-dimensional arrangement of particles called a lattice. There are four types: giant ionic (NaCl, MgO: ions held by ionic bonds), giant metallic (Cu: cations in delocalised electrons), giant molecular or covalent (diamond, graphite, graphene, SiO2: atoms joined by covalent bonds throughout) and simple molecular (I2, ice, C60: molecules held by weak intermolecular forces). To explain a property, name the particles, the force that must be overcome, and how strong it is.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Four ways to build a solid.. Every crystalline solid is a lattice, but what sits at the lattice points and what holds them together changes everything. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: The four lattices | Rotate each lattice in the simulator: sodium chloride, copper, diamond and iodine. Project tab 4 of the lab (States of matter lab, lattices) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Carbon, four ways | Switch to the carbon allotropes. Project tab 5 of the lab (States of matter lab, carbon allotropes) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Ice, iodine and alloys | Finish with the metals and alloys simulator: push a layer of pure metal, then add larger or smaller atoms and push again. Project tab 6 of the lab (States of matter lab, metals and alloys) and let a student drive it while the class predicts. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Ice, iodine and alloys, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| For a giant molecular solid, melting breaks covalent bonds; for a simple molecular solid it overcomes only intermolecular forces. | Mixing these up is the most common error in this section. Graphite conducts through delocalised electrons, not ions. |
Lattice structures and properties: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
Name the type of lattice in each of: (a) magnesium oxide, (b) copper, (c) graphite, (d) ice.
Explain why silicon(IV) oxide has a much higher melting point than carbon dioxide, although both are oxides of Group 14 elements.
Explain why graphite conducts electricity and is used as a lubricant, but diamond does neither.
Iodine is a shiny dark solid that turns to a purple vapour on gentle warming. Explain this in terms of its structure.
Explain why steel, an alloy of iron with carbon, is harder than pure iron.
Substance X melts at 801 °C, does not conduct as a solid but conducts when molten, and dissolves in water. Substance Y melts at 114 °C and does not conduct in either state. Identify the type of lattice in each and justify your answers.
Answer key and marking notes
- (a) giant ionic.
- (b) giant metallic.
- (c) giant molecular (covalent).
- (d) simple molecular (hydrogen-bonded).
- SiO2 is giant molecular: every Si is covalently bonded to four O throughout the structure.
- Melting it requires many strong covalent bonds to be broken.
- CO2 is simple molecular; melting or subliming it overcomes only weak id-id forces between molecules.
- In graphite each C forms three covalent bonds; the fourth electron is delocalised and can move along the layers.
- The layers are held by weak forces, so they slide easily.
- In diamond each C forms four covalent bonds, leaving no delocalised electrons, and the rigid 3D network cannot slide.
- Solid iodine is a simple molecular lattice of I2 molecules held by id-id forces.
- These forces are weak, so little energy is needed to separate the molecules; the I–I bonds are not broken.
- The carbon atoms are a different size from the iron atoms and disrupt the regular layers.
- The layers can no longer slide over each other easily, so the alloy is harder.
- X is giant ionic: high melting point from strong ionic bonding; conducts only when molten because the ions become mobile; soluble in water.
- Y is simple molecular: low melting point because only weak intermolecular forces are overcome.
- Y contains no ions or delocalised electrons, so it never conducts (it could be iodine).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which substance has a giant covalent (giant molecular) structure?
- Asilicon(IV) oxide, SiO2
- Biodine, I2
- Ccarbon dioxide, CO2
- Dmagnesium oxide, MgO
The diagram shows a side view of the layers in graphite. Why does graphite conduct electricity along the layers?
- Aeach carbon atom forms three covalent bonds, and the fourth outer electron is delocalised and free to move along the layer
- Bthe layers can slide over each other, carrying charge with them
- Cgraphite contains C4+ ions that move when a voltage is applied
- Deach carbon atom forms four covalent bonds, which allows electrons to pass from atom to atom
Solid iodine turns into a vapour on gentle warming. What is overcome when this happens?
- Athe weak instantaneous dipole-induced dipole forces between I2 molecules
- Bthe covalent bonds between the two iodine atoms in each molecule
- Cthe electrostatic attraction between I+ and I− ions
- Dthe attraction between iodine ions and delocalised electrons
Why is ice less dense than liquid water at 0 °C?
- Ain ice each molecule is hydrogen bonded to four others in a tetrahedral arrangement, giving an open lattice with a lot of empty space
- Bice contains air bubbles trapped when the water freezes
- Cthe covalent O–H bonds become longer when water freezes
- Din ice the hydrogen bonds are broken, so the molecules move further apart
The diagram shows the arrangement of atoms in a pure metal (A) and in an alloy (B). Why is the alloy harder than the pure metal?
- Athe different-sized atoms disrupt the regular layers, so the layers cannot slide over each other easily
- Bthe alloy has more delocalised electrons, so it has covalent bonds between the atoms
- Cthe larger atoms form ionic bonds with the metal ions around them
- Dthe alloy has no delocalised electrons, so the atoms are held in fixed positions
Substance Z has a melting point of 1710 °C. It does not conduct electricity as a solid or when molten, and it is insoluble in water and in hexane. What is the most likely structure of Z?
- Agiant covalent
- Bgiant ionic
- Cgiant metallic
- Dsimple molecular
Diamond and buckminsterfullerene, C60, are both forms of pure carbon. Diamond does not melt below about 3500 °C, but C60 sublimes at about 600 °C. Which is the best explanation?
- AC60 is simple molecular, so only the intermolecular forces between separate C60 molecules are overcome, whereas melting diamond breaks strong covalent bonds throughout a giant lattice
- Bthe covalent bonds in C60 are much weaker than those in diamond because the molecule is curved
- CC60 has delocalised electrons that weaken its bonding, whereas diamond has none
- Ddiamond is ionic, while C60 is covalent
In the giant covalent structure of silicon(IV) oxide, SiO2, how many oxygen atoms are bonded to each silicon atom, and how many silicon atoms are bonded to each oxygen atom?
- A4 oxygen atoms per silicon; 2 silicon atoms per oxygen
- B2 oxygen atoms per silicon; 1 silicon atom per oxygen
- C4 oxygen atoms per silicon; 4 silicon atoms per oxygen
- D6 oxygen atoms per silicon; 3 silicon atoms per oxygen
Quiz key
SiO2 is a giant covalent lattice in which every silicon atom is bonded to four oxygen atoms and every oxygen to two silicon atoms, so there are no discrete molecules and the melting point is very high (about 1710 °C). Iodine is a simple molecular solid: I2 molecules held in a lattice by weak instantaneous dipole-induced dipole forces. Carbon dioxide is also simple molecular, which is why it is a gas at room temperature even though carbon and silicon are in the same group. Magnesium oxide is a giant ionic lattice of Mg2+ and O2− ions, not a covalent network.
Within each layer every carbon is covalently bonded to three others in hexagonal rings, leaving one outer electron per atom that is delocalised over the whole layer; these mobile electrons carry the current along the layers. The layers sliding (held only by weak forces) explains why graphite is soft and slippery, not why it conducts. Graphite has no ions. A carbon with four covalent bonds, as in diamond, has no free electrons, which is why diamond is an insulator.
Solid iodine is a lattice of I2 molecules held together only by instantaneous dipole-induced dipole (London) forces, so little energy is needed to separate the molecules and it sublimes on gentle warming; the vapour is still I2 molecules. The I–I covalent bond stays intact; breaking it would produce iodine atoms and needs much more energy. Iodine is an element with no ions, so there is no ionic lattice. It is not a metal and has no delocalised electrons.
In ice every water molecule forms four hydrogen bonds (two through its hydrogen atoms and two through its lone pairs) arranged tetrahedrally, which holds the molecules in a rigid, open, hexagonal lattice with large gaps. On melting some hydrogen bonds break, the lattice collapses and the molecules pack more closely, so the liquid is denser. Pure ice is still less dense than water, so trapped air is not the reason. The O–H covalent bonds do not change significantly on freezing. Hydrogen bonds are formed, not broken, when water freezes.
In a pure metal the identical ions form regular layers that can slide past each other when a force is applied, so the metal is malleable and fairly soft. In an alloy, atoms of a different size distort the layers so they no longer line up, and more force is needed to make them slide; the alloy is harder and stronger. The bonding in the alloy is still metallic, not covalent or ionic. An alloy still has delocalised electrons, which is why alloys such as brass and steel conduct electricity.
A very high melting point shows that many strong bonds must be broken, and no conduction in any state shows there are no mobile ions or delocalised electrons; insolubility in both polar and non-polar solvents fits a covalent network. These are the properties of a giant covalent structure such as SiO2. A giant ionic solid would conduct when molten (and is often soluble in water). A giant metallic solid conducts as a solid. A simple molecular substance would have a low melting point because only weak intermolecular forces need to be overcome.
Buckminsterfullerene consists of separate, ball-shaped C60 molecules held together by instantaneous dipole-induced dipole forces; subliming it separates the molecules without breaking any C–C bonds, so relatively little energy is needed. Diamond is a giant covalent lattice, so every atom is held by four strong covalent bonds and a huge amount of energy is needed. The C–C bonds inside C60 are strong covalent bonds and are not broken on subliming, so their strength is not the explanation. Delocalised electrons in C60 do not weaken the intramolecular bonding, and they play no part in separating molecules. Diamond is covalent, not ionic; it contains only carbon atoms.
Each silicon atom forms four single bonds, all to oxygen, in a tetrahedral arrangement like the carbon atoms in diamond, and each oxygen forms two bonds, both to silicon, bridging two tetrahedra. The ratio 4 : 2 gives one Si to two O, matching SiO2. 2 and 1 describes a separate O=Si=O molecule like CO2, which is not the structure of silicon dioxide. 4 and 4 would give a 1 : 1 formula and would need oxygen to form four bonds. 6 and 3 is the coordination in some ionic oxides such as TiO2, not the covalent network in SiO2.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.