Periodic Trends: Atomic Number Increases Across a Period

Steps:

• Across a period, elements gain protons, increasing atomic number from left to right.
• Neutral atoms gain electrons to match protons, so total electrons rise.
• Electron shells stay constant within one period (same energy level).
• Metallic character decreases as nonmetals dominate rightward.

Why D is correct:

• Atomic number (protons, property 3) increases sequentially; electrons (property 4) equal protons in neutral atoms, per the definition of atomic structure.

Why the others are wrong:

• A: Metallic character (1) decreases; shells (2) remain fixed.
• B: Metallic character (1) decreases; protons (3) increase.
• C: Shells (2) remain fixed; electrons (4) increase.

Final answer: D