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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
The modern periodic table is arranged by atomic number, i.e. the number of protons in the nucleus. Atomic number uniquely identifies each element. Early tables used atomic mass, which caused problems because elements with the same proton number but different neutron numbers (isotopes) have different masses. Consequently, elements such as tellurium and iodine were placed in the wrong groups. Since isotopes have the same proton number, ordering by atomic number gives a consistent and reliable arrangement of elements.
Examiner tips
- State that the table is arranged by atomic number and explain why this is reliable; mention isotopes and the mass anomalies; keep the answer concise and use the term ‘atomic number’
- Use the word ‘protons’ to show understanding of the definition of atomic number
Common mistakes
- Confusing atomic number with atomic mass; writing that the table is arranged by mass; not mentioning isotopes or the mass anomalies
Mark scheme (5 marks)
- The modern periodic table arranges elements in order of atomic (proton) number / number of protons
- The atomic number equals the number of protons in the nucleus of an atom
- Ordering by atomic mass caused problems / placed some elements in the wrong group (e.g. anomalies such as tellurium and iodine)
- Isotopes of the same element have different masses (different numbers of neutrons) but the same proton number
- Because isotopes exist, atomic mass is not a reliable way to order elements, but proton number uniquely identifies each element
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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