Across a period from left to right, the atomic radius generally
Answer and explanation
Correct answer: C. decreases
- Step 1: As one moves horizontally from left to right across a period, valence electrons are sequentially added to the same principal energy shell.
- Step 2: Simultaneously, each successive element adds one additional proton to the nucleus, causing the effective nuclear charge () to increase steadily.
- Step 3: This heightened inward electrostatic pull contracts the electron orbitals closer to the nucleus, causing the atomic radius to systematically decrease.
Answer
decreases.
Why not the others: increases describes the periodic trend down a vertical group, where additional electron shells are added; first increases then decreases contradicts the steady increase in effective nuclear charge across representative periods; stays the same ignores electrostatic shell contraction.
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