Which periodic trend correctly explains why atomic radius decreases from left to right across Period 3 of the periodic table?
Answer and explanation
Correct answer: D. Effective nuclear charge increases while electrons are added to the same principal energy level
- Step 1: Across a period, each successive element adds a proton to the nucleus and an electron to the same valence shell ().
- Step 2: Because shielding from core electrons remains essentially constant, the effective nuclear charge () increases, drawing the valence electrons closer to the nucleus and decreasing atomic radius.
Answer
Effective nuclear charge increases while electrons are added to the same principal energy level.
Why not the others: 'Number of inner shells increases' occurs down a group, not across a period; 'Electronegativity decreases' is false (it increases across a period); 'Shielding increases significantly' is incorrect because valence electrons do not effectively shield each other.
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