Concept:Across a period, elements are arranged by increasing atomic number, and their valence electrons increase progressively from left to right.
Explanation:Option A is false: elements in the same group have the same number of valence electrons, not the same number of electron shells.
Option B is false: across a period, non-metallic properties generally increase, not decrease.
Option C is false: elements in the same period have the same number of electron shells, not the same number of valence electrons.
Option D is true: as you move across a period, the number of valence electrons increases progressively from 1 to 8.
Answer:D. The valence electrons of the elements increase progressively across the period.