Concept:Across a period, elements show a progressive increase in valence electrons from left to right.
Explanation:• Option A is incorrect: non-metallic properties increase, not decrease, across a period.
• Option B is correct: valence electrons increase progressively across each period.
• Option C is incorrect: elements in the same group have the same number of valence electrons, not the same number of electron shells.
• Option D is incorrect or incomplete because elements in the same period do not have the same number of valence electrons.
Answer:B. The valence electrons of the elements increase progressively across the period