The shape of the lesson
By the end of the lesson, learners can
- Down the group: Move from magnesium to barium in the simulator and compare the size of each atom and how tightly its outer electrons are held.
- With oxygen, water and acids: Run each reaction in the simulator and compare the vigour from magnesium to barium.
- Radius up, IE down, reactivity up.
- Water: M + 2H2O → M(OH)2 + H2.
- Mg + steam → MgO + H2.
- Oxidation number always +2.
One picture every idea returns to
All Group 2 elements have two outer s electrons and form M2+ ions, so their compounds have oxidation number +2. Down the group the atoms gain shells, so atomic radius increases and shielding increases; the outer electrons are less strongly held, so the first and second ionisation energies decrease. Less energy is needed to form M2+, so reactivity increases from Mg to Ba. The metals burn in oxygen to form MO, react with water to give hydroxides and hydrogen (Mg only slowly unless it is steam), and react with dilute acids to form salts and hydrogen.
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, More reactive the lower you go.. Magnesium barely reacts with cold water; barium fizzes vigorously. 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: Down the group | Move from magnesium to barium in the simulator and compare the size of each atom and how tightly its outer electrons are held. Project tab 1 of the lab (Group 2 lab, down the group) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: With oxygen, water and acids | Run each reaction in the simulator and compare the vigour from magnesium to barium. Project tab 2 of the lab (Group 2 lab, reactions) and let a student drive it while the class predicts. | Jigsaw |
| 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 With oxygen, water and acids, 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.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
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 |
|---|---|
| Calcium, strontium and barium react with dilute sulfuric acid at first but soon slow down, because their sulfates are insoluble and coat the metal. | Do not use this to argue that reactivity decreases down the group. And magnesium with steam gives the oxide, not the hydroxide. |
Group 2 metals and their reactions: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why the first ionisation energy decreases from magnesium to barium.
Write equations for the reactions of (a) strontium with water, (b) magnesium with steam and (c) calcium with dilute hydrochloric acid.
Describe what is seen when magnesium ribbon is burned in air, and write the equation.
Calculate the volume of hydrogen at room conditions formed when 0.200 g of calcium reacts with excess water. (Ar Ca 40.1)
Barium oxide is added to water and the solution tested with universal indicator. Write the equation and predict the colour.
Explain why the reaction of calcium with dilute sulfuric acid slows down quickly, while its reaction with dilute hydrochloric acid continues.
Answer key and marking notes
- The atoms have more electron shells, so the outer electron is further from the nucleus.
- There is more shielding by inner electrons.
- These outweigh the increase in nuclear charge, so less energy is needed to remove the outer electron.
- (a) Sr + 2H2O → Sr(OH)2 + H2.
- (b) Mg + H2O(g) → MgO + H2.
- (c) Ca + 2HCl → CaCl2 + H2.
- A very bright white flame and a white powder (ash) is left.
- 2Mg + O2 → 2MgO.
- n(Ca) = 0.200 ÷ 40.1 = 4.99 × 10−3 mol = n(H2).
- V = 4.99 × 10−3 × 24.0 = 0.120 dm3 (120 cm3).
- BaO + H2O → Ba(OH)2.
- Ba(OH)2 is quite soluble and strongly alkaline, so the indicator turns purple (pH about 13 to 14).
- Calcium sulfate is only sparingly soluble, so it forms a coating on the calcium that keeps the acid away.
- Calcium chloride is soluble, so the metal surface stays exposed.
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.
How does the reactivity of the Group 2 elements change from magnesium to barium, and why?
- Areactivity increases, because the outer electrons are further from the nucleus and more shielded, so less energy is needed to remove them
- Breactivity decreases, because larger atoms hold their outer electrons less tightly
- Creactivity increases, because the nuclear charge increases down the group
- Dreactivity stays the same, because every element forms an ion with a 2+ charge
The graph shows the first ionisation energies of the Group 2 elements from magnesium to barium. Which statement explains the trend?
- Athe outer electron is in a shell further from the nucleus and is more shielded by inner electrons, so it is attracted less strongly
- Bthe nuclear charge decreases down the group, so the outer electron is attracted less strongly
- Cthe outer electron is in a p subshell for the heavier elements, which is higher in energy than s
- Dthe ionisation energy decreases because the atoms become smaller down the group
Which statement correctly describes the reaction of magnesium with water?
- Amagnesium reacts only very slowly with cold water, but burns in steam to give magnesium oxide and hydrogen
- Bmagnesium reacts vigorously with cold water to give magnesium hydroxide and hydrogen
- Cmagnesium does not react with steam because it is the least reactive Group 2 metal
- Dmagnesium reacts with steam to give magnesium hydroxide and oxygen
Which equation correctly represents the reaction of calcium with cold water?
- ACa + 2H2O → Ca(OH)2 + H2
- BCa + H2O → CaO + H2
- C2Ca + 2H2O → 2CaOH + H2
- DCa + 2H2O → Ca(OH)2 + O2
Pieces of barium of the same size are added to dilute hydrochloric acid and to dilute sulfuric acid of the same concentration of H+. The reaction with sulfuric acid soon slows down and almost stops. Why?
- Ainsoluble barium sulfate forms a layer on the metal surface, stopping the acid from reaching it
- Bsulfuric acid is a weaker acid than hydrochloric acid, so it reacts more slowly
- Cbarium becomes less reactive once it has formed Ba2+ ions
- Dbarium sulfate is very soluble, so it uses up the water and the acid becomes too concentrated
1.373 g of barium reacts completely with cold water. What volume of hydrogen, measured at room conditions, is produced? (Ar Ba = 137.3; molar gas volume = 24.0 dm3 mol−1)
- A240 cm3
- B480 cm3
- C120 cm3
- D0.240 cm3
A white solid oxide of a Group 2 element is shaken with water and universal indicator is added; the solution turns purple. A second sample of the oxide dissolves in dilute hydrochloric acid. Which pair of equations is correct for strontium oxide?
- ASrO + H2O → Sr(OH)2; SrO + 2HCl → SrCl2 + H2O
- BSrO + H2O → Sr + H2O2; SrO + 2HCl → SrCl2 + H2
- CSrO + H2O → Sr(OH)2; SrO + HCl → SrCl + OH−
- DSrO2 + H2O → Sr(OH)2 + O2; SrO + 2HCl → SrCl2 + H2O
Barium has a nuclear charge of +56 whereas magnesium has only +12, yet barium loses its outer electrons much more easily. Which explanation is best?
- Abarium's extra protons are almost cancelled by the extra inner shells of electrons, so the effective attraction on the outer 6s electrons is similar, but they are much further from the nucleus
- Bbarium's outer electrons are in a d subshell, which is easier to ionise than an s subshell
- Cthe larger nuclear charge of barium repels its outer electrons, pushing them away
- Dbarium has fewer outer electrons than magnesium, so they are easier to remove
Quiz key
Each Group 2 metal reacts by forming M2+. Down the group the atomic radius increases and there are more inner shells shielding the outer s electrons, so the ionisation energies fall and the metals lose their electrons more easily: reactivity increases (Ba reacts far more vigorously with water than Mg). Saying reactivity decreases is the classic error: electrons held less tightly makes an atom MORE reactive, not less. The nuclear charge does increase, but on its own that would make the electrons harder to remove; it is outweighed by the extra distance and shielding. All the elements form 2+ ions, but the energy needed to form them differs, so the reactivity is not the same.
The first IE falls from 738 kJ mol−1 (Mg) to 503 kJ mol−1 (Ba). Each element has one more occupied shell than the one above, so the outer s electron is further from the nucleus and shielded by more inner electrons; the extra nuclear charge is outweighed, and less energy is needed to remove it. The nuclear charge increases down a group (12 protons in Mg, 56 in Ba), it does not decrease. The outer electrons of every Group 2 element are in an s subshell (3s, 4s, 5s, 6s), never p. The atoms get larger down the group, not smaller.
Magnesium has the highest ionisation energies in the group, so it reacts only very slowly with cold water (a few bubbles over days). With steam it reacts readily, glowing brightly: Mg + H2O(g) → MgO + H2. Vigorous reaction with cold water is the classic error; that describes Ca, Sr and Ba, increasingly so down the group. Being the least reactive in Group 2 does not stop magnesium reacting with steam. With steam the product is the oxide, not the hydroxide, and the gas given off is hydrogen, not oxygen.
Calcium, strontium and barium react with cold water to give the hydroxide and hydrogen: M + 2H2O → M(OH)2 + H2, with fizzing that becomes more vigorous down the group. The oxide is formed only when magnesium reacts with steam, not when calcium reacts with cold water. CaOH would need calcium in the +1 state, but Group 2 metals always have oxidation number +2. The gas is hydrogen, produced when water is reduced; oxygen is not released.
With hydrochloric acid the product, BaCl2, is soluble and the reaction continues. With sulfuric acid the product BaSO4 is insoluble, so it coats the barium and prevents further contact with the acid (the same effect occurs, less strongly, with CaSO4 on calcium). Sulfuric acid is a strong acid, and the H+ concentrations were the same. Forming ions does not change the reactivity of the remaining metal. BaSO4 is insoluble, not very soluble; that is why it is used as a test precipitate for sulfate.
Ba + 2H2O → Ba(OH)2 + H2. n(Ba) = 1.373 / 137.3 = 0.0100 mol, and the ratio Ba : H2 is 1 : 1, so n(H2) = 0.0100 mol and V = 0.0100 × 24 000 = 240 cm3. 480 cm3 assumes two moles of H2 per mole of Ba, confusing the 2H2O with the hydrogen. 120 cm3 halves the ratio, as if H2 came from half the metal. 0.240 cm3 uses 24.0 dm3 but then labels the answer in cm3; the correct value is 0.240 dm3 = 240 cm3.
The oxide ion is a base. With water it forms the hydroxide, SrO + H2O → Sr(OH)2, an alkaline solution (purple with universal indicator, pH about 13). With an acid it gives a salt and water only: SrO + 2HCl → SrCl2 + H2O. The second option has the oxide decomposing to the metal and hydrogen gas forming from an oxide; hydrogen is formed only when the METAL reacts with an acid. SrCl would need Sr in the +1 state, but Group 2 elements are always +2. SrO2 is a peroxide; the normal oxide formed by Group 2 metals is MO, from 2M + O2 → 2MO.
Barium has four more inner shells than magnesium. These inner electrons shield the outer pair from most of the extra nuclear charge, so the net attraction felt by the outer 6s electrons is similar to that on Mg's 3s electrons, while the distance from the nucleus is much larger. The attraction is therefore weaker and the ionisation energies are lower (503 against 738 kJ mol−1 for the first IE), so barium is more reactive. Barium's outer electrons are 6s2, not d. A positive nucleus attracts electrons; it never repels them. Every Group 2 element has exactly two outer electrons.
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.