Mendeleev arranged elements in his early periodic table based on 'atomic weights'. Explain why, in some cases, this led to elements being placed in the wrong order.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When Mendeleev published his periodic table, he arranged elements in order of increasing atomic weight. He also left gaps in his table where he predicted undiscovered elements would fit.
Model answer (4 marks)
Mendeleev used the measured atomic weights of elements to order them. However, atoms of the same element can have different numbers of neutrons, giving rise to isotopes. The different isotopes have slightly different masses, so the average atomic weight of an element depends on which isotopes are present and their relative abundances. Because atomic weight is not a simple function of the number of protons, the sequence based on atomic weight did not always match the true order based on atomic number. Consequently some elements were placed in the wrong position or group in Mendeleev’s table.
Examiner tips
- Mention isotopes and their effect on atomic weight
- Explain that atomic weight ≠ atomic number
- Show why this caused mis‑ordering
- Use the exact wording from the mark scheme
Common mistakes
- Confusing atomic weight with atomic number
- Ignoring the role of isotopes
- Failing to explain that the mis‑order is due to differing neutron numbers
Mark scheme (4 marks)
- Different atoms of the same element can have different numbers of neutrons
- These atoms are called isotopes
- Isotopes have different masses / atomic weights, so the average atomic weight of an element is affected by which isotopes are present and their abundance
- This meant the order based on atomic weight did not always match the correct order based on atomic number (number of protons), placing some elements in the wrong position / group
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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