Periodic trends in atomic structure across a period

Steps:

• Across a period, elements fill the same electron shell, so number of shells (2) stays constant.
• Atomic number increases left to right, raising protons (3) and electrons (4) in neutral atoms.
• Metallic character (1) decreases due to stronger nuclear pull on electrons, making elements less metallic.
• Thus, only protons and electrons increase consistently.

Why D is correct:

• In a period, atomic number (protons) rises sequentially, and neutral atoms gain matching electrons per the definition of atomic number Z = protons = electrons.

Why the others are wrong:

• A: Metallic character decreases; shells constant.
• B: Metallic character decreases; protons increase.
• C: Shells constant; electrons increase.

Final answer: D