Chemistry • Year 11 • Module 1 • Lesson 15
The Periodic Table: Organisation
Build HSC Band 5–6 extended-response technique on evaluating periodic organisation, applying trend reasoning to data, and designing an investigation based on the periodic table.
1. Data + scenario: Mendeleev’s prediction of eka-silicon (Band 5–6)
8 marks Band 5–6
Scenario. In 1871, Mendeleev used the pattern of his periodic table to predict the existence and properties of an undiscovered element he called “eka-silicon” (Es). In 1886, the German chemist Clemens Winkler discovered germanium (Ge, Z = 32) and found its properties remarkably close to Mendeleev’s predictions. The table below compares Mendeleev’s predictions with the experimentally determined values for germanium.
| Property | Mendeleev’s prediction (1871) | Winkler’s measured value (1886) |
|---|---|---|
| Relative atomic mass | 72 | 72.6 |
| Density (g cm−3) | 5.5 | 5.35 |
| Colour / appearance | Grey metal | Greyish-white metal |
| Oxide formula | Es2O3 → EsO2 | GeO2 |
| Chloride formula | EsCl4 | GeCl4 |
| Chloride boiling point | Below 100 °C | 83.1 °C |
| Oxide density (g cm−3) | 4.7 | 4.70 |
Sources: Mendeleev (1871) Über die Beziehungen der Eigenschaften zu den Atomgewichten der Elemente; Winkler (1886) Journal für praktische Chemie. Illustrative comparison.
Q1. Evaluate how well Mendeleev’s model of the periodic table is supported by the discovery of germanium. In your response you must:
- State the group and period of germanium and explain what these reveal about its electron configuration, using the data in the table as supporting evidence.
- Assess the accuracy of Mendeleev’s predictions by comparing at least three properties from the table quantitatively or qualitatively.
- Explain how Mendeleev was able to predict properties of an unseen element, linking your answer to the concept of periodicity and the use of surrounding elements in the table.
- Identify one property where Mendeleev’s prediction was less accurate and suggest a reason.
- Reach an evidence-based judgement on whether the discovery of germanium strengthens or weakens confidence in the periodic table as a predictive tool.
2. Experimental design — testing the Group 17 reactivity trend (Band 5–6)
7 marks Band 5–6
Research question. The periodic table predicts that halogens (Group 17) become less reactive going down the group. A Year 11 student claims that “chlorine is more reactive than bromine and iodine because it is at the top of the group.” Design a scientific investigation to test this claim using displacement reactions between halogen solutions and halide solutions.
Constraints: You have access to standard Year 11 laboratory equipment. Available reagents include: aqueous solutions of Cl2(aq), Br2(aq), I2(aq), NaCl(aq), NaBr(aq), NaI(aq), and cyclohexane (organic solvent to show colour). Maximum two weeks investigation time.
Q2. Design the investigation and present it in the format below.
- State a testable hypothesis identifying the independent and dependent variables.
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps, including how you will use cyclohexane to detect halogen displacement.
- Construct a results table showing the combinations you would test and predict the expected colour change in the cyclohexane layer for each.
- State what result would falsify your hypothesis.
- Identify two limitations of your design and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Group, period and electron configuration: Germanium (Z = 32) is in Group 14, Period 4, p-block. Period 4 means it has 4 occupied electron shells. Group 14 means it has 4 valence electrons. The data supports this: GeCl4 and GeO2 both reflect 4 bonds — consistent with 4 valence electrons and the formation of 4 covalent bonds [1].
Accuracy assessment (three properties): (1) Atomic mass: predicted 72, actual 72.6 — error of 0.8%, extremely close [1]. (2) Density: predicted 5.5 g cm−3, actual 5.35 g cm−3 — error of 2.7%, within experimental uncertainty of the era [1]. (3) Chloride boiling point: predicted “below 100 °C”, actual 83.1 °C — qualitatively correct and consistent with Mendeleev’s reasoning that a low-boiling covalent chloride would form. Accept any three valid comparisons from the table [1 per property, max 3].
How Mendeleev predicted: Mendeleev used the concept of periodicity: properties of elements repeat at regular intervals when arranged by atomic mass. He interpolated the properties of the gap element from its four nearest neighbours — silicon (Group 14, Period 3, above) and tin (Group 14, Period 5, below), plus arsenic (Group 15, Period 4) and gallium (Group 13, Period 4) on either side. By averaging and extrapolating from these known values, he estimated density, atomic mass, oxide/chloride formulas, and boiling point [1].
Less accurate property: Mendeleev initially predicted the oxide formula as Es2O3 (sesquioxide) before correcting it to EsO2; the actual oxide is GeO2, reflecting 4-valence-electron chemistry, not 3. This initial error arose because Mendeleev was uncertain whether eka-silicon would behave more like aluminium (Group 13, forms M2O3) or silicon (Group 14, forms MO2) [1].
Evaluative judgement: The discovery of germanium strongly strengthens confidence in the periodic table as a predictive tool. Six of the seven properties match within 3% or qualitatively agree. The ability to forecast density, atomic mass, oxide formula, chloride formula, and boiling point of an element not yet discovered from table position alone is extraordinary. The one initial formula error was subsequently corrected, and the corrected prediction (EsO2) was exact. This demonstrates that the periodic table captures a deep underlying pattern in elemental properties driven by electron configuration [1].
Marking criteria (8 marks): 1 = correct group (14), period (4), and block (p-block) with electron configuration link to the formula data; 1 = property 1 with quantitative/qualitative comparison; 1 = property 2 with comparison; 1 = property 3 with comparison; 1 = explanation of how Mendeleev used periodicity and interpolation from neighbouring elements; 1 = identifies a less accurate property with a reasonable explanation; 1 = explicit evidence-based judgement (strengthens confidence); 1 = consistent use of precise chemical terminology throughout (periodicity, valence electrons, interpolation, effective nuclear charge, period, group).
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If halogen reactivity decreases going down Group 17 (Cl > Br > I), then a more reactive halogen will displace a less reactive halide ion from solution, producing a colour change in cyclohexane, while a less reactive halogen will not displace a more reactive halide. Independent variable: the halogen and halide combination used. Dependent variable: whether a colour change occurs in the cyclohexane layer (orange/brown/purple). [1]
Variables: IV = halogen used (Cl2, Br2, I2) + halide tested (Cl−, Br−, I−). DV = colour change in cyclohexane layer. Controlled: concentration of all solutions (0.1 mol L−1), volume of halogen added (2 mL), volume of halide used (5 mL), volume of cyclohexane added (2 mL), temperature (room temperature, ~22 °C). [1]
Procedure: (1) Label 6 test tubes for each combination: Cl2/NaBr, Cl2/NaI, Br2/NaCl, Br2/NaI, I2/NaCl, I2/NaBr. (2) Add 5 mL of halide solution to each test tube. (3) Add 2 mL of the corresponding halogen solution to each test tube; mix gently for 30 seconds. (4) Add 2 mL of cyclohexane to each tube, stopper and shake gently; allow layers to separate and record the colour of the upper cyclohexane layer: Cl2(cyclohexane) = pale yellow, Br2 = orange, I2 = purple/violet; no colour = no displacement. [1]
Results table (predicted):
| Halogen added | Halide tested | Predicted cyclohexane colour | Displacement? |
|---|---|---|---|
| Cl2 | NaBr | Orange | Yes (Cl > Br) |
| Cl2 | NaI | Purple/violet | Yes (Cl > I) |
| Br2 | NaCl | Colourless | No (Br < Cl) |
| Br2 | NaI | Purple/violet | Yes (Br > I) |
| I2 | NaCl | Colourless | No (I < Cl) |
| I2 | NaBr | Colourless | No (I < Br) |
Award 1 mark for a complete, correct predicted results table with at least four combinations. [1]
Falsification: The hypothesis would be falsified if I2 produced a colour change when added to NaCl(aq) or NaBr(aq) (indicating I displaced Cl− or Br−), which would contradict the prediction that reactivity decreases down Group 17. [1]
Limitations: (1) Halogen solutions can oxidise slowly in air, reducing concentration over time — freshly prepared solutions should be used but ageing may introduce error [1]. (2) The cyclohexane layer colour may be ambiguous for dilute solutions (pale colours hard to distinguish), reducing reliability of displacement detection [1].
Improvement: Repeat each combination three times and use a colourimetry reading instead of visual colour comparison to improve reliability and reduce subjective interpretation. [1]
Marking criteria (7 marks): 1 = testable hypothesis with IV and DV named; 1 = procedure with four clear steps including cyclohexane layer colour test; 1 = complete predicted results table (at least 4 combinations with correct predictions); 1 = falsification statement; 1 = one valid limitation; 1 = second valid limitation; 1 = one specific reliability improvement with justification.