Question detail
Which element in Period 3 has the highest first ionisation energy?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Atomic structure
Question
- A. Sodium
- B. Magnesium
- C. Silicon
- D. Argon
Answer
Argon
Explanation
Argon has the highest first ionisation energy in Period 3 because it has a complete outer shell, making it more stable and requiring more energy to remove an electron.
Common mistake
Misinterpreting ionisation energy trends
Students often think that ionisation energy increases steadily across a period and decreases down a group, ignoring the effect of subshell filling and electron shielding.
Explain that ionisation energy rises across a period due to increasing nuclear charge and decreasing atomic radius, but drops when a new subshell begins (e.g., from Na to Mg). Down a group, ionisation energy decreases because added electrons are farther from the nucleus and more shielded, reducing the energy required to remove an electron.
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