Periodic Table Ordered by Atomic Number

Steps:

• Recall that the modern periodic table arranges elements by increasing atomic number.
• Atomic number equals the number of protons in the nucleus.
• This ordering groups elements with similar properties in periods and groups.
• Historical versions used atomic mass, but Mendeleev's table was refined to atomic number by Moseley.

Why C is correct:

• Atomic number (protons) defines an element's identity and determines its position, as per the periodic law.

Why the others are wrong:

• A: Outer electrons influence reactivity but not ordering.
• B: Neutrons affect isotopes, not the table's sequence.
• D: Atomic masses were used historically but cause anomalies like argon-potassium reversal; protons provide the precise order.

Final answer: C