The shape of the lesson
By the end of the lesson, learners can
- Reading the clues: Use the trends simulator as a reference table while you work through each clue below.
- Beyond Period 3: Apply the same reasoning to elements in other periods.
- Formulae give the oxidation number and group.
- Ionisation energy jump gives the number of outer electrons.
- Structure from melting point and conductivity together.
- Oxide/chloride with water shows metal or non-metal.
One picture every idea returns to
The trends seen in Period 3 recur in every period. To place an unknown element, use several clues together: the formulae of its oxide and chloride give its oxidation number and so its group; successive ionisation energies jump after the last outer electron; a very high melting point and non-conducting behaviour point to a giant covalent structure; oxide and chloride behaviour with water (basic, amphoteric, acidic, hydrolysed) shows where it sits between metal and non-metal.
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, Use the pattern to predict.. The Periodic Table repeats itself. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Reading the clues | Use the trends simulator as a reference table while you work through each clue below. Project tab 1 of the lab (Periodicity lab, trends across period 3) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Beyond Period 3 | Apply the same reasoning to elements in other periods. | Cooperative Concept Mapping |
| 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, 14 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 Beyond Period 3, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| Do not base an identification on a single property. | Melting point alone cannot separate a strong metal from a giant covalent solid; combine it with conductivity, oxide behaviour and ionisation energy data. And remember that the dips in first ionisation energy (at Groups 13 and 16) repeat in every period. |
Chemical periodicity of other elements: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Element A: m.p. 660 °C, good conductor, oxide reacts with both HCl and NaOH, chloride gives pH 3 in water. Identify A and explain each piece of evidence.
The first five ionisation energies of element B are 578, 1817, 2745, 11 577 and 14 842 kJ mol−1. Deduce the group of B and the formula of its oxide.
Element C forms a white solid oxide that dissolves in water to give pH 2, and a chloride CCl5. Identify C, and write equations for its oxide and chloride reacting with water.
Predict, with a reason, whether GeO2 is more or less acidic than SiO2.
Suggest why first ionisation energy drops from nitrogen to oxygen in Period 2.
Element D has m.p. −101 °C, does not conduct, and its hydride dissolves in water to give a strongly acidic solution. Identify D and its structure.
Answer key and marking notes
- A is aluminium.
- Moderate melting point and good conductivity: giant metallic structure.
- Amphoteric oxide and partly hydrolysed chloride: aluminium's intermediate position in Period 3.
- The big jump is between the 3rd and 4th, so B has three outer electrons: Group 13.
- Its oxide has the formula X2O3; B is aluminium, so Al2O3.
- C is phosphorus (oxidation number +5).
- P4O10 + 6H2O → 4H3PO4.
- PCl5 + 4H2O → H3PO4 + 5HCl.
- Less acidic (GeO2 is amphoteric).
- Metallic character increases down Group 14, so the oxide becomes more basic.
- Oxygen's electron is removed from a 2p orbital containing a pair of electrons.
- Repulsion between the paired electrons makes it easier to remove, as with P and S in Period 3.
- D is chlorine.
- Simple molecular Cl2, held by weak id-id forces; HCl is a strong acid in water.
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.
Element X is in Period 3. Its oxide has a very high melting point, is insoluble in water and reacts with hot concentrated sodium hydroxide but not with hydrochloric acid. Its chloride is a liquid that fumes in moist air, forming a white solid and an acidic solution. What is X?
- Asilicon
- Baluminium
- Cphosphorus
- Dmagnesium
The oxide of a Period 3 element Y is insoluble in water but dissolves both in dilute hydrochloric acid and in aqueous sodium hydroxide. What is the formula of the chloride of Y?
- AAlCl3 (exists as Al2Cl6 in the vapour)
- BMgCl2
- CSiCl4
- DPCl5
The first five successive ionisation energies of an element, in kJ mol−1, are 578, 1817, 2745, 11577 and 14842. In which group of the Periodic Table is the element?
- Agroup 13
- Bgroup 14
- Cgroup 15
- Dgroup 2
Periodicity repeats in each period. Silicon has the highest melting point in Period 3. Which element in Period 2 would you predict to have the highest melting point?
- Acarbon, because like silicon it is in group 14 and forms a giant covalent structure
- Bnitrogen, because it is next to carbon and has more protons
- Clithium, because metals have higher melting points than non-metals
- Dberyllium, because it has the strongest metallic bonding in Period 2
A Period 3 element Z has a melting point of 650 °C. Its oxide is slightly soluble in water giving a pH of about 9, and its chloride has formula ZCl2 and gives a pH of about 6.5 in water. What is Z?
- Amagnesium
- Bsodium
- Caluminium
- Dsulfur
Germanium is in group 14, directly below silicon. Using the trends in Period 3, which is the best prediction for the chloride of germanium?
- AGeCl4, a simple molecular liquid that is hydrolysed by water to give an acidic solution
- BGeCl4, an ionic solid that dissolves in water to give a neutral solution
- CGeCl2, an ionic solid with a high melting point, like magnesium chloride
- DGeCl3, which dimerises like aluminium chloride
The first ionisation energies, in kJ mol−1, of eight elements with consecutive proton numbers are:
A 1012, B 1000, C 1251, D 1521, E 419, F 590, G 633, H 659.
Which element is in group 1?
- AE
- BD
- CB
- DA
Period 3 element Q forms an oxide in which Q has its maximum oxidation number of +5, and a chloride with five chlorine atoms per molecule. The oxide reacts with water to give an acidic solution of pH about 2. How many moles of sodium hydroxide react with the acid formed from 1.00 mol of the oxide, assuming complete neutralisation?
- A12.0
- B4.00
- C10.0
- D6.00
Quiz key
A high-melting, water-insoluble oxide that reacts only with alkali is SiO2 (giant covalent, acidic). Its chloride SiCl4 is a simple molecular liquid that hydrolyses completely: SiCl4 + 2H2O → SiO2 + 4HCl, the white solid being SiO2. Aluminium oxide also has a high melting point and is insoluble, but it reacts with hydrochloric acid too (amphoteric), and AlCl3 is a solid that sublimes. Phosphorus oxide, P4O10, has a low melting point and reacts with water. Magnesium oxide is basic, reacting with acid, and MgCl2 is a high-melting ionic solid.
Reacting with both acid and alkali means the oxide is amphoteric, which in Period 3 identifies Al2O3, so Y is aluminium and its chloride is AlCl3 (aluminium is +3, group 13; the chloride dimerises to Al2Cl6). Magnesium oxide is basic: it reacts with acid but not with sodium hydroxide. Silicon dioxide is acidic: it reacts only with (hot concentrated) alkali. Phosphorus(V) oxide reacts with water to give phosphoric acid, so it is not insoluble in water.
The big jump comes between the 3rd (2745) and 4th (11577) ionisation energies: the 4th electron is removed from an inner shell, much closer to the nucleus and less shielded. So there are three outer-shell electrons, putting the element in group 13 (it is aluminium, which also forms Al3+ and Al2O3). Group 14 would need four electrons before the jump. Group 15 would show the big jump only after the 5th ionisation energy, but here the 4th and 5th values are both huge. Group 2 would show the jump after the 2nd ionisation energy, but 2745 is only a modest increase on 1817.
Carbon is directly above silicon in group 14. As diamond or graphite it has a giant covalent structure, so, just like silicon in Period 3, it has the highest melting point in its period (over 3500 °C). Nitrogen exists as small N2 molecules with weak forces between them, the Period 2 equivalent of phosphorus, and is a gas at room temperature. Lithium is the Period 2 equivalent of sodium, with weak metallic bonding (melting point about 181 °C); the chart shows that metals are not automatically higher-melting than giant covalent non-metals. Beryllium has stronger metallic bonding than lithium, as Mg does compared with Na, but metallic bonding still gives a lower melting point than a giant covalent lattice.
A melting point of 650 °C, a slightly soluble basic oxide (MgO, pH about 9) and a chloride MgCl2 that gives a very slightly acidic solution (pH about 6.5, because Mg2+ is only slightly hydrolysed) all point to magnesium. Sodium melts at 98 °C, forms NaCl (not ZCl2), and its oxide gives pH 13 to 14. Aluminium forms AlCl3, which gives pH about 3, and its oxide is insoluble. Sulfur does form a chloride SCl2, but it melts at 115 °C and its oxides are acidic, not basic.
Elements in the same group show the same pattern. Silicon forms SiCl4, a covalent liquid (group 14, oxidation number +4) that hydrolyses to an acidic solution, so the prediction for germanium is GeCl4, a covalent liquid that hydrolyses: in fact GeCl4 boils at about 84 °C and fumes in moist air. An ionic chloride giving a neutral solution is typical of group 1 (NaCl), not group 14. GeCl2 with a formula like MgCl2 would put germanium in group 2. GeCl3 would be the group 13 pattern shown by aluminium, not the group 14 pattern.
The largest fall is from D (1521) to E (419). D has the highest value, so it is a noble gas with a full outer shell; the next element, E, has a single electron in a new shell further from the nucleus and well shielded, so E is in group 1 (these are P, S, Cl, Ar, K, Ca, Sc, Ti). D is the noble gas, group 18, the maximum before the fall, not group 1. B shows the small dip due to paired p electrons, so it is in group 16 (sulfur). A is just before that dip, the p3 element of group 15 (phosphorus); a low value alone does not make an element group 1, it must follow the noble-gas peak.
Oxidation number +5 and a pentachloride means Q is phosphorus (group 15), so the oxide is P4O10: P4O10 + 6H2O → 4H3PO4. Each H3PO4 is tribasic, so 4 × 3 = 12.0 mol NaOH. 4.00 treats H3PO4 as monobasic. 10.0 counts the oxygen atoms in the formula of the oxide instead of the acidic hydrogens. 6.00 is the number of moles of water that react with the oxide, which is not the amount of alkali neutralised.
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.