Chemistry • Year 11 • Module 1 • Lesson 17
Periodic Trends: Atomic Radius
Build HSC advanced extended-response technique by evaluating data, designing investigations, and constructing multi-criteria arguments about atomic size trends.
1. Data + scenario: isoelectronic species in industrial chemistry (Evaluate)
8 marks Evaluate
Scenario. In some Australian industrial electrolysis processes, solutions containing isoelectronic species, ions with the same number of electrons, are compared. The table below shows four isoelectronic ions all with 18 electrons (the argon electron configuration). Despite having the same electron count, their ionic radii are different. (Argon, though also isoelectronic with 18 electrons, is left out of this ionic-radius comparison because a noble-gas atom is sized by a van der Waals radius, which is not directly comparable to an ionic radius.)
| Species | Symbol | Nuclear charge (protons) | Electrons | Ionic radius (pm) |
|---|---|---|---|---|
| Sulfide ion | S2− | 16 | 18 | 184 |
| Chloride ion | Cl− | 17 | 18 | 181 |
| Potassium ion | K+ | 19 | 18 | 138 |
| Calcium ion | Ca2+ | 20 | 18 | 100 |
Ionic radius data adapted from Shannon (1976), Acta Crystallographica A32: 751–767. Illustrative values.
Q1. Analyse and evaluate the data above to explain why isoelectronic species with 18 electrons have different radii. In your response you must:
- Identify the trend in ionic radius as nuclear charge increases among these 18-electron species and describe it quantitatively using data from the table.
- Explain the trend in terms of effective nuclear charge: refer to what changes and what stays the same across these species.
- Distinguish between the radii of the anions (S2−, Cl−) compared to their parent neutral atoms (S: 104 pm, Cl: 99 pm), explaining what this tells us about the effect of electron gain on size.
- Evaluate the claim: “K+ is smaller than Cl− because it has fewer electrons.” Assess whether this is a complete or incomplete explanation.
- State one limitation of using ionic radius alone to predict reactivity in solution.
2. Experimental design, testing whether atomic radius predicts reactivity in Group 1 (Evaluate)
7 marks Evaluate
Background. The lesson states that smaller atoms generally hold outer electrons more tightly, which should mean they are less reactive in reactions that involve losing electrons (e.g. reaction of Group 1 metals with water). This suggests the reactivity of Group 1 metals should increase down the group as atomic radius increases: Li < Na < K.
Research question. Design a scientific investigation to test whether the reactivity of Group 1 metals with water increases in order Li < Na < K, using observable evidence to rank reactivity.
Constraints (safety): Reacting Group 1 metals with water is a teacher-demonstration-only reaction, potassium in particular can ignite the hydrogen it produces, so you must not handle Group 1 metals yourself. Design your investigation around evidence you can collect safely: observations of a teacher demonstration (behind a safety screen), a recorded video you can replay frame by frame, and/or a published secondary dataset of Group 1 reactivity. You have access to: a stopwatch, a thermometer, universal indicator (added by the teacher), video playback, and reference data tables.
Q2. Design the investigation and present it in the format below.
- State a testable hypothesis that links atomic radius to reactivity (include a predicted ranking).
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe, in at least four numbered steps, how you will observe and record the teacher demonstration / video (and/or analyse the secondary dataset) to measure or rank reactivity quantitatively or semi-quantitatively, without handling the metals yourself.
- Explain what result would falsify your hypothesis.
- State two limitations of your design and one way to improve reliability.
Q1, Sample Band 6 response (8 marks), annotated
Trend identification and quantitative description: Among the four isoelectronic ions (all 18 electrons), ionic radius decreases as nuclear charge increases: S2− (184 pm) → Cl− (181 pm) → K+ (138 pm) → Ca2+ (100 pm). (Argon is isoelectronic too, but its van der Waals radius is not directly comparable to ionic radii, so it is excluded from this trend.) The clear trend is that ionic radius decreases from S2− (184 pm) to Ca2+ (100 pm) as nuclear charge rises from 16 to 20, a decrease of 84 pm for 4 additional protons. [1, trend with quantitative reference to data]
Explanation via effective nuclear charge: All five species contain 18 electrons in identical shell configurations. What differs is the nuclear charge (number of protons). A higher nuclear charge exerts a stronger attractive force on the same 18 electrons, pulling them closer to the nucleus and reducing the ionic radius. Shielding is essentially constant across these species because the electron count and arrangement are fixed. Therefore, as nuclear charge increases from 16 (S2−) to 20 (Ca2+), effective nuclear charge increases, and each successive species is smaller. [1, Zeff explanation; 1, shielding constant / electron count fixed]
Anions compared to parent atoms: S2− (184 pm) is much larger than neutral S (104 pm), and Cl− (181 pm) is larger than neutral Cl (99 pm). In both cases, gaining electrons adds to electron–electron repulsion in the outer shell without adding protons. The same nuclear charge must now attract more electrons; the effective nuclear charge per electron decreases, so the electron cloud expands. This confirms that electron gain always increases atomic/ionic radius. [1, anion vs atom comparison with data; 1, mechanism (repulsion, no extra protons)]
Evaluation of the K+ vs Cl− claim: The claim that “K+ is smaller than Cl− because it has fewer electrons” is incomplete and partly incorrect. K+ and Cl− have the same number of electrons (18). The reason K+ is smaller (138 pm vs 181 pm) is that K+ has more protons (19 vs 17): a higher nuclear charge pulling the same electron count more strongly. Saying K+ “has fewer electrons” is factually wrong for this comparison; the correct explanation is higher effective nuclear charge in K+. [1, identification that electron count is equal; 1, correct explanation referencing higher Z/Zeff in K+]
Limitation: Ionic radius alone does not predict reactivity in solution because charge density (charge÷size), hydration energy, and the nature of the chemical interaction all play important roles. For example, a smaller, highly charged ion may actually be less reactive toward certain ligands because it is too strongly hydrated. [1, valid limitation beyond simple size]
Marking criteria summary (8 marks): 1 = identifies decreasing trend with quantitative data reference; 1 = explains via effective nuclear charge increasing with proton number; 1 = notes shielding/electron count constant; 1 = compares anion to parent atom with data; 1 = explains mechanism of anion expansion (repulsion, no new protons); 1 = identifies claim as incorrect/incomplete (same electron count); 1 = provides correct Zeff explanation for K+ vs Cl−; 1 = states a valid limitation of using ionic radius alone.
Q2, Sample Band 6 response (7 marks), annotated
Hypothesis: If atomic radius increases down Group 1 (Li < Na < K), then K will react most vigorously with water because its outer electron is held least tightly by the nucleus, followed by Na, then Li. Independent variable: identity of the Group 1 metal (Li, Na, K) used in the teacher demonstration/video. Dependent variable: observed rate/vigour of reaction (bubbles counted per 10 s on video replay, time for the metal to be fully consumed, or temperature rise read from a safe distance; rated slow/moderate/vigorous). Controlled variables (kept equal by the teacher): mass of metal, water volume, water temperature (~20 °C). [1, testable hypothesis with IV and DV]
Procedure (student observes and analyses; the teacher performs the reaction behind a safety screen, or a recorded video is used): (1) Before the demonstration, set up an observation table for the three metals (Li, Na, K) with columns for bubbling rate, indicator colour change, metal movement, and whether the reaction self-sustains. (2) The teacher adds equal masses of Li, Na and K to equal volumes of water containing universal indicator; the student records, from a safe distance or via video, the start time for each. (3) Observe and record: (a) speed of bubbling (count bubbles per 10 s using video replay), (b) colour change of indicator (monitors H2/OH− production), (c) movement of metal across the surface, (d) whether the reaction is self-sustaining or ignites. (4) From the video/demonstration, record the time for each metal to be consumed and the indicator’s final colour, then rank the three reactions. The student designs and analyses the protocol but never handles the metals. [1, four steps including semi-quantitative reactivity measure]
Falsification: If Li reacts more vigorously than Na or K (or if there is no clear ranking Li < Na < K), the hypothesis would be falsified, atomic radius alone would not explain the reactivity trend. [1]
Limitations: (1) A small demonstration sample may not give clearly distinguishable differences for the less reactive Li, and a single demonstration is not replicated. [1] (2) Subjectivity in rating “vigour” of bubbling introduces observer bias; two different students may rank the same reaction differently, which is why frame-by-frame video evidence is used. [1]
Improvement: Use several repeats of the demonstration (or multiple standardised video recordings) with the same mass and water volume to calculate an average bubble count or temperature rise, improving reliability. Where available, compare against published secondary reactivity data to validate the observed ranking. [1]
Expected outcome: The demonstration/video should show K reacting most vigorously (the hydrogen produced can ignite, which is exactly why this is a teacher-only reaction), Na moderately, and Li most slowly, consistent with increasing atomic radius and decreasing hold on the outer electron down Group 1. [1, prediction linked to atomic radius reasoning]
Marking criteria summary (7 marks): 1 = hypothesis linking atomic radius to reactivity with predicted ranking; 1 = four procedural steps including a semi-quantitative reactivity measure; 1 = falsification condition stated; 1 = first limitation; 1 = second limitation; 1 = improvement to reliability; 1 = precise chemical terminology (effective nuclear charge, outer electron, independent/dependent/controlled variable, Group 1 reactivity trend).