Periodic Table Organization by Groups, Periods, and Atomic Number

Steps:

• Identify groups as vertical columns defined by similar valence electron counts.
• Recognize periods as horizontal rows based on the number of electron shells.
• Note that elements are ordered left to right and top to bottom by increasing atomic number (protons).
• Compare options to match these definitions exactly.

Why A is correct:

• Matches the standard definition: groups by outer shell electrons (valence electrons determine chemical properties), periods by occupied shells (energy levels), and arrangement by proton number (atomic number for stability and order).

Why the others are wrong:

• B: Incorrectly swaps group and period definitions; uses mass number instead of proton number.
• C: Correctly defines group and period but wrongly uses mass number for arrangement.
• D: Incorrectly swaps group and period definitions despite correct arrangement by proton number.

Final answer: A