The periodic classification is an arrangement of the elements

1. Analyze the question

The question asks for the fundamental property used to arrange elements in the modern Periodic Table. You need to distinguish between the historical method (Mendeleev's approach) and the modern method (Moseley's approach).

2. Explain the concept

• Historical Context: Early chemists like Dmitri Mendeleev arranged elements based on increasing atomic weights (or atomic masses). However, this led to several inconsistencies where elements did not fit perfectly into groups with similar chemical properties.
• Modern Periodic Law: In 1913, Henry Moseley discovered that the properties of elements are a periodic function of their atomic numbers (the number of protons in the nucleus). This discovery corrected the flaws in the earlier tables and is the basis for the modern Periodic Table we use today.

3. Show the solution

• Option A (Atomic weights): This was used in the early versions of the periodic table but was found to be inaccurate for predicting chemical behavior.
• Option B (Isotopic weights): Isotopes of the same element have different weights but identical chemical properties. Arranging by isotopic weights would be chaotic and unscientific.
• Option C (Molecular weights): Molecular weights apply to compounds or molecules, not individual elements in the periodic table.
• Option D (Atomic numbers): The modern Periodic Table is strictly organized by increasing atomic number ($Z$). This ensures that elements with the same number of valence electrons (which determine chemical reactivity) fall into the same vertical groups.

4. Identify the answer

The correct basis for the periodic classification of elements is the atomic number.

5. Exam Tip

Memory Trick: Remember "Moseley = Modern = Number."

• Moseley (the scientist) gave us the Modern table based on the atomic Number.
• If you see a question about the original or early periodic table, the answer is Atomic Weight. If it refers to the modern periodic table, the answer is always Atomic Number.