# Explain why the periodic table is arranged in the order it is today, rather than in the order originally proposed by early scientists such as Mendeleev.

> AQA A-Level Physics (7408) — 3.2.1 Particles · Explain · 5 marks

> Early versions of the periodic table arranged elements in order of atomic mass. Modern chemists and physicists now understand the internal structure of the atom.

## Mark scheme (5 marks)

1. The modern periodic table arranges elements in order of atomic (proton) number / number of protons
2. The atomic number equals the number of protons in the nucleus of an atom
3. Ordering by atomic mass caused problems / placed some elements in the wrong group (e.g. anomalies such as tellurium and iodine)
4. Isotopes of the same element have different masses (different numbers of neutrons) but the same proton number
5. Because isotopes exist, atomic mass is not a reliable way to order elements, but proton number uniquely identifies each element

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