Periodic Trends

AP Chemistry· difficulty 2/5

A student plots empirical atomic radii of period-3 elements (Na through Cl) versus atomic number and observes a steady decrease.

The graph shows that atomic radius decreases across period 3. The best explanation is:

A
$Shielding increases faster than nuclear charge$
B
$Effective nuclear charge increases while shielding stays roughly constant$
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C
$The number of energy levels increases$
D
$Electrons are added to inner shells$

Explanation

Across a period, protons are added but new electrons enter the same shell, so Zeff increases and the valence electrons are pulled in tighter.

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