CAPF AC 2021 Q30: Science & Technology – Chemistry
- (a) Ionization energy
- (b) Electron affinity
- (c) Electronegativity
- (d) Atomic radius
Answer: (d) Atomic radius
Explanation
Periodic trends
How to think: the question wants a property that falls across a period and rises down a group.
That is the pattern of size.
The other three properties follow the opposite pattern.
| Property | Left to right in a period | Top to bottom in a group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionization energy | Increases | Decreases |
| Electron affinity | Increases (generally) | Decreases (generally) |
| Electronegativity | Increases | Decreases |
Why size behaves this way: across a period the nuclear charge grows but the shell stays the same, so electrons are pulled closer.
Down a group a new shell is added each time, so the atom grows.
Answer: (d) Atomic radius.
Why each option is right or wrong
- (a) Incorrect. Ionization energy increases across a period and decreases down a group.
- (b) Incorrect. Electron affinity generally increases across a period and decreases down a group.
- (c) Incorrect. Electronegativity increases across a period and decreases down a group.
- (d) CORRECT. Atomic radius decreases left to right and increases top to bottom.
Key point
Atomic radius and metallic character: decrease across a period, increase down a group. Ionization energy, electron affinity, electronegativity and non-metallic character: the opposite.
Remember
Size shrinks across, grows down.
Common mistakes
- Thinking atoms get bigger across a period because electrons are added.
- Forgetting that a new shell is added at each step down a group.
- Not seeing that three of the four options share the same trend.
Exam tip
Educated guess: ionization energy, electron affinity and electronegativity all move together. Three options with the same trend cannot each be the single answer, so the odd one, atomic radius, is correct.
Sources
- NCERT Class X Science — Periodic Classification of Elements (trends in the modern periodic table)
- NCERT Class XI Chemistry — Classification of Elements and Periodicity in Properties