Question detail
What happens to the atomic radius of elements as you move across Period 3 from sodium to argon?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Periodicity
Question
- A. It decreases due to increased nuclear charge.
- B. It increases due to increased electron shielding.
- C. It remains constant across the period.
- D. It increases due to the addition of electron shells.
Answer
It decreases due to increased nuclear charge.
Explanation
The correct option is It decreases due to increased nuclear charge.. It decreases due to increased nuclear charge. is the best answer because it directly supports the AQA A-Level Chemistry objective to explain the trend in atomic radius across Period 3. This reasoning is anchored to Physical properties of Period 3 elements in Periodicity, and it separates Period 3 from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.
Common mistake
Confusing Atomic Radius Trends
Students often incorrectly state that atomic radius decreases across Period 3 due to increased electron shielding rather than increased nuclear charge.
To explain the trend in atomic radius across Period 3, use the following reasoning: The atomic radius decreases as you move from sodium (Na) to argon (Ar) because the proton number increases, leading to a greater nuclear charge that pulls the electrons closer to the nucleus, outweighing any effects of electron shielding. Therefore, the atomic radius trend can be summarized as follows: 1. **Formula/Rule**: Atomic radius decreases across a period due to increased nuclear charge. 2. **Substitution**: As you move from Na (11 protons) to Ar (18 protons), the nuclear charge increases from +11 to +18. 3. **Working**: The increased nuclear charge results in a stronger attraction between the nucleus and the electron cloud, leading to a smaller atomic radius. 4. **Answer**: The atomic radius decreases across Period 3. 5. **Units/Conclusion**: The trend is observed in picometers (pm) or nanometers (nm), confirming that atomic radius decreases as you move from left to right across the period.
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