Answer: A. Across Period 3 from sodium (metallic) to argon (noble gas), the elements transition from metals → metalloids → non-metals → noble gas. Metallic character decreases.
B is wrong: every element in Period 3 has exactly 3 filled shells (the first three energy levels are involved). The number of filled shells stays the same.
C is wrong: the number of outer-shell electrons increases from 1 (Na) to 8 (Ar) across the period — that's the definition of going across a period.
D is wrong: the proton number increases steadily from 11 (Na) to 18 (Ar) across the period — it doesn't decrease.
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