Mendeleev arranged the elements in order of atomic weights and left gaps in his periodic table. Explain why Mendeleev left gaps in his periodic table and why his ordering of some elements was not quite correct.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Mendeleev left gaps because many elements had not yet been discovered, so the table could be expanded later.
The gaps allowed elements with similar chemical properties to be placed in the same group or column.
He used the gaps to predict the properties of the missing or undiscovered elements.
The ordering was not quite correct because atoms of the same element can have different numbers of neutrons – these are isotopes.
Because isotopes exist, the average atomic weight of an element does not always place it in the correct position when ordered by atomic weight alone.
The gaps allowed elements with similar chemical properties to be placed in the same group or column.
He used the gaps to predict the properties of the missing or undiscovered elements.
The ordering was not quite correct because atoms of the same element can have different numbers of neutrons – these are isotopes.
Because isotopes exist, the average atomic weight of an element does not always place it in the correct position when ordered by atomic weight alone.
Examiner tips
- Use the word ‘gaps’ and explain discovery of elements; mention property similarity; note prediction ability; mention isotopes and average atomic weight; keep answer concise and use exact terminology.
- Structure answer in short points to cover all five marks.
Mark scheme (5 marks)
- Mendeleev left gaps because some elements had not yet been discovered / found
- The gaps allowed elements with similar chemical properties to be placed in the same group / column
- Mendeleev was able to predict the properties of the missing / undiscovered elements using the gaps
- The ordering was not quite correct because atoms of the same element can have different numbers of neutrons — these are called isotopes
- Because isotopes exist, the average atomic weight of an element does not always place it in the correct position when ordered by atomic weight alone
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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