Question detail
What is the trend in first ionisation energy across Period 3?
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 increases from sodium to argon.
- B. It decreases from sodium to argon.
- C. It remains constant across Period 3.
- D. It fluctuates without a clear trend.
Answer
It increases from sodium to argon.
Explanation
The correct option is It increases from sodium to argon.. It increases from sodium to argon. is the best answer because it directly supports the AQA A-Level Chemistry objective to explain the trend in first ionisation energy across Period 3. This reasoning is anchored to Physical properties of Period 3 elements in Periodicity, and it separates first ionisation energy 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
Misunderstanding Ionisation Energy Trends
Students often confuse the trend in first ionisation energy across Period 3 with atomic radius, thinking that ionisation energy decreases as atomic radius increases.
To clarify, remember that first ionisation energy increases across Period 3 due to increased nuclear charge attracting electrons more strongly. Use the formula for ionisation energy trends: as atomic number increases, ionisation energy increases because of greater effective nuclear charge. Therefore, the correct trend is that first ionisation energy increases across Period 3.
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