The position of an element in the Periodic Table is determined by?

Step 1: Analyze the question The question asks what fixes an element's position (its period and group) in the modern Periodic Table.

Step 2: The concept The modern Periodic Table is arranged by atomic number, which is the number of protons in the nucleus of an atom. Henry Moseley showed this in 1913. Before that, Mendeleev arranged elements by relative atomic mass, which caused some mismatches (for example, tellurium and iodine).

• The number of protons gives the atomic number.
• In a neutral atom, the number of protons equals the number of electrons.
• The electron arrangement determines the period (number of shells) and the group (outer-shell electrons).

Step 3: Check each option

• A. Atomic radius: This is a periodic property that changes because of the position. It does not determine the position.
• B. Density: This is a physical property and varies with conditions. It does not determine the position.
• C. Relative atomic mass: This was Mendeleev's basis, but the modern table does not use it. Some elements with higher mass come before elements with lower mass, such as argon and potassium.
• D. Number of protons: This is the atomic number, the basis of the modern Periodic Table. ✔
• E. Number of neutrons: Isotopes of the same element have different neutron numbers but occupy the same position, so neutrons cannot determine position.

Answer: D, the number of protons in its atom.

Exam tip: Remember "Position = Proton count" (atomic number). If a question asks about the modern Periodic Table, think atomic number. If it asks about Mendeleev's table, think atomic mass.