UPSC Prelims 2001 Q121: Science & Technology – Chemistry
Consider the following statements with reference to the Periodic Table of chemical
elements:
I. Ionisation potential gradually decreases along a period.
II. In a group of elements, electron affinity decreases as the atomic weight increases.
III. In a given period, electronegativity decreases as the atomic number increases.
Which of these statement(s) is/are correct?
- (a) I only
- (b) II only
- (c) I and III
- (d) II and III
Answer: (b) II only
Explanation
Statements on trends in the Periodic Table
| Statement | Verdict |
|---|---|
| I. Ionisation potential gradually decreases along a period | Wrong — it generally increases across a period |
| II. In a group, electron affinity decreases as atomic weight increases | Correct (general trend) — larger atoms attract an extra electron less |
| III. In a period, electronegativity decreases as atomic number increases | Wrong — it increases across a period |
| Reason for period trends | Nuclear charge rises while electrons are added to the same shell |
Across a period atoms get smaller and hold electrons more tightly, so ionisation potential and electronegativity increase. Down a group atoms get larger, so electron affinity generally decreases. Only statement II is correct.
Answer: (b) II only.
Why each option is right or wrong
- (a) Incorrect. Statement I is wrong; ionisation potential rises across a period.
- (b) CORRECT. Only the group trend in electron affinity is right.
- (c) Incorrect. Both I and III state the period trends the wrong way round.
- (d) Incorrect. Statement III is wrong; electronegativity rises across a period.
Key point
Exceptions: chlorine has higher electron affinity than fluorine; nitrogen's ionisation energy exceeds oxygen's.
Remember
IE and electronegativity rise across a period; electron affinity falls down a group.
Common mistakes
- Reversing the period trends of IE and EN.
- Thinking electronegativity falls across a period.
- Forgetting atoms shrink across a period.
Exam tip
Across a period: size down, everything else (IE, EA, EN) up.
Sources
- NCERT Class XI — Chemistry Part I, chapter 'Classification of Elements and Periodicity in Properties'
- J. D. Lee — Concise Inorganic Chemistry, chapter on periodic trends