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What happens to the atomic radius as you move across a period from left to right?
AIt increases
BIt stays fixed
CIt decreases
DIt doubles
Answer & Solution
Correct answer: C. It decreases
1. Moving across a period adds protons to the nucleus while filling the same energy level.
2. The stronger positive pull draws the electron cloud in tighter.
3. The atomic radius therefore decreases across a period.
4. Ionisation energy and electronegativity move the other way and increase across a period.
5. Down a group the pattern flips, since the atomic radius increases.
_Source: Siyavula Physical Sciences Grade 10 (Everything Science, CC BY 4.0), Chapter 5: The periodic table_
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