The shape of the lesson
By the end of the lesson, learners can
- Reactions with water: Each alkali metal reacts with water in the same way, but the violence increases down the group.
- Trends and predictions: Step down Group I in the simulator and watch the atoms get bigger.
- Water: metal hydroxide + hydrogen.
- Melting point decreases down the group.
- Reactivity increases down the group.
- Reason: outer electron further out, lost more easily.
One picture every idea returns to
The Group I alkali metals are soft, have relatively low melting points and low densities, and are good conductors. They react with cold water to form an alkaline hydroxide and hydrogen: 2Na + 2H2O → 2NaOH + H2. Down the group, the melting point decreases, the density generally increases and the reactivity increases. Their outer electron is further from the nucleus and more easily lost.
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, Soft metals that fizz.. Lithium, sodium and potassium are metals you can cut with a knife and store under oil. 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: Reactions with water | Each alkali metal reacts with water in the same way, but the violence increases down the group. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Trends and predictions | Step down Group I in the simulator and watch the atoms get bigger. Project tab 8 of the lab (Atomic structure lab, radius trends) and let a student drive it while the class predicts. | Jot Thoughts |
| 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, 15 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 Trends and predictions, 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.
Jot Thoughts
Teams write ideas on slips, one idea per slip, as many as they can in two minutes, saying each aloud as they place it in the middle. Then they sort the slips, which is where the structure of the topic appears.
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 |
|---|---|
| Reactivity increases down Group I but decreases down Group VII. | Do not carry the Group I trend over to the halogens. And the solution formed with water is alkaline because of the hydroxide, not because of the hydrogen gas. |
Group I: the alkali metals: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Describe three observations when a small piece of sodium is added to a trough of water.
Write a word equation and a balanced symbol equation for the reaction of potassium with water.
State three physical properties shared by the Group I metals.
Rubidium is below potassium in Group I. Predict (a) its melting point compared with potassium, (b) how it reacts with water, and (c) the formula of its hydroxide.
Explain why the solution left after sodium reacts with water turns universal indicator purple.
Explain, in terms of electrons, why reactivity increases down Group I.
Answer key and marking notes
- It floats.
- It melts into a ball and moves around on the surface.
- It fizzes (bubbles of gas) and gets smaller until it disappears.
- potassium + water → potassium hydroxide + hydrogen
- 2K + 2H2O → 2KOH + H2
- Soft (can be cut with a knife).
- Relatively low melting points.
- Low densities (lithium, sodium and potassium float on water); good conductors of electricity.
- (a) lower than potassium (below 63 °C).
- (b) more violently than potassium, even explosively, giving hydrogen.
- (c) RbOH.
- Sodium hydroxide is formed, which dissolves to give OH− ions.
- The solution is strongly alkaline (high pH).
- Down the group the outer electron is in a shell further from the nucleus and is more shielded by inner shells.
- It is attracted less strongly, so it is lost more easily when the metal reacts.
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 physical property is shared by lithium, sodium and potassium?
- AThey are soft enough to be cut with a knife.
- BThey are dense enough to sink in water.
- CThey melt only above 500 °C.
- DThey are poor conductors of electricity.
The melting points of lithium, sodium and potassium are 181 °C, 98 °C and 63 °C. Rubidium is the next element down Group I. What is the most likely melting point of rubidium?
- A39 °C
- B72 °C
- C105 °C
- D210 °C
A small piece of lithium is added to a trough of cold water. Which observation is expected?
- AIt floats and fizzes steadily, slowly getting smaller.
- BIt sinks to the bottom and gives off a lilac flame.
- CIt melts into a ball and the gas produced catches fire.
- DIt explodes as soon as it touches the water.
A gas is given off when sodium reacts with water. Which test identifies this gas?
- AIt gives a squeaky pop with a lighted splint.
- BIt relights a glowing splint.
- CIt turns limewater milky.
- DIt bleaches damp litmus paper.
Potassium reacts with water and universal indicator is then added to the solution. Which pH value best describes the solution formed?
- ApH 13
- BpH 7
- CpH 5
- DpH 1
A student writes: "The solution left after sodium reacts with water is alkaline because hydrogen gas is produced." Which statement correctly corrects the student?
- AThe solution is alkaline because sodium hydroxide dissolves to give hydroxide ions.
- BThe solution is alkaline because the hydrogen dissolves to give hydrogen ions.
- CThe solution is neutral because sodium chloride is formed.
- DThe solution is acidic because hydrogen ions are formed.
Lithium sulfide has the formula Li2S. What is the most likely formula of rubidium sulfide?
- ARb2S
- BRbS
- CRbS2
- DRb2S3
Francium is at the bottom of Group I. Which statement best explains why chemists predict that francium reacts with chlorine to form FrCl?
- AA francium atom has one outer-shell electron, which it loses to form Fr+.
- BA francium atom has seven outer-shell electrons, which it shares with chlorine.
- CFrancium has the same number of electron shells as chlorine.
- DFrancium is the least reactive element in Group I.
Quiz key
The alkali metals are soft and can be cut with a knife to show a shiny surface. Lithium, sodium and potassium have low densities and float on water, so they do not sink. Their melting points are low (all below 200 °C), and like other metals they conduct electricity well.
The melting point decreases down Group I, so rubidium should melt below potassium's 63 °C; its actual melting point is about 39 °C. A value of 72 °C is above potassium, 105 °C is above sodium, and 210 °C would mean the trend had reversed and increased down the group.
Lithium is the least reactive of the alkali metals: it floats (low density) and fizzes steadily as hydrogen is given off, gradually disappearing. It never sinks. Melting into a ball is seen with sodium, and hydrogen igniting (with a lilac flame) is seen with potassium. An explosive reaction is typical of metals further down, such as rubidium and caesium.
The gas is hydrogen, which burns with a squeaky pop when a lighted splint is held near it. Relighting a glowing splint is the test for oxygen, turning limewater milky is the test for carbon dioxide, and bleaching damp litmus paper is the test for chlorine.
Potassium hydroxide dissolves to give a strongly alkaline solution with a pH well above 7, such as 13, so the indicator turns purple. pH 7 would be neutral, and pH 5 and pH 1 would be acidic; a metal hydroxide solution is never acidic.
Hydrogen escapes as a gas; the product that stays dissolved is sodium hydroxide, which releases hydroxide ions (OH−) and makes the solution alkaline. Hydrogen ions would make a solution acidic, not alkaline, and no chloride is present, so sodium chloride cannot form.
Rubidium, like lithium, has one outer-shell electron, so it forms a 1+ ion and reacts in the same way. Sulfur (Group VI) forms S2−, so two Rb+ ions balance one S2− ion, giving Rb2S. RbS would need a Rb2+ ion, typical of Group II. RbS2 and Rb2S3 do not balance the charges of Rb+ and S2−.
Every Group I atom has one outer-shell electron, and chemical reactions involve the outer electrons. Francium therefore loses one electron to form Fr+, which combines with Cl− as FrCl, just like NaCl and KCl. Seven outer electrons describes a halogen, not an alkali metal. Francium has many more shells than chlorine, and as the lowest Group I element it is predicted to be the most reactive, not the least.
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.