alleryWhy does atomic radius decrease across a period?
AElectrons are added to new outer shells
BNuclear charge rises with same shell count
CAtomic mass falls and electrons sink
DAtoms become more metallic and denser
Answer & Solution
Correct answer: B. Nuclear charge rises with same shell count
1. Across a period, every new element adds one proton and one electron.
2. The added electron enters the same outermost shell.
3. So the principal quantum number of the valence electron does not change.
4. Increased nuclear charge pulls the same outer shell inward more tightly.
5. Hence atomic radius shrinks across a period.
6. Option A is the opposite case; C and D are unrelated.
_Source: Selina Concise Chemistry ICSE Class 10, Ch 1 'Periodic Table' (aplustopper.com extract)_
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