UPSC Prelims 2003 Q105: Science & Technology – Chemistry
Assertion (A): In the periodic table of chemical elements, electron affinity is always found
to increase from top to bottom in a group.
Reason (R): In a group, the atomic radius generally increases from top to bottom.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is NOT the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (d) A is false but R is true
Explanation
Electron affinity in the periodic table
| Part | Verdict |
|---|---|
| A. Electron affinity always increases from top to bottom in a group | False: it generally decreases down a group, with exceptions such as F and Cl |
| R. Atomic radius generally increases from top to bottom | True |
A is false because electron affinity does not always increase down a group (it tends to fall), while R is true.
Answer: (d) A is false but R is true.
Why each option is right or wrong
- (a) Incorrect. A is false, so R cannot explain it.
- (b) Incorrect. A is false, as electron affinity does not always rise.
- (c) Incorrect. R is true, so this code is wrong.
- (d) CORRECT. A is false because electron affinity generally falls down a group, while R is true.
Key point
Down a group: atomic radius up, ionisation energy down, electron affinity generally down.
Remember
Electron affinity generally decreases down a group.
Common mistakes
- Believing that electron affinity rises with atomic size.
- Mixing it up with the metallic character of elements.
- Forgetting the chlorine-fluorine exception.
Exam tip
Bigger atoms attract an extra electron less strongly.
Sources
- NCERT Class XI Chemistry — Periodicity
- J. D. Lee — Concise Inorganic Chemistry (Periodicity)