Mendeleev arranged the elements into a periodic table. Describe two features of Mendeleev's arrangement of the elements and explain why his ordering was not always correct.

Eduqas GCSE Chemistry — 2.5 The production of important chemicals · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Mendeleev arranged the elements in order of increasing atomic mass.
Elements with similar chemical properties were placed in the same group or column.
He left gaps in the table where elements had not yet been discovered.
His ordering was not always correct because elements with the same chemical properties can have different atomic masses due to isotopes, so atomic mass alone does not always reflect the true periodic trend.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. ‘in order of increasing atomic mass’ and ‘same group or column’. Explain the gaps as ‘where elements had not yet been discovered’. Mention isotopes as the reason for incorrect ordering.
  • common_mistakes
  • :
  • Using ‘atomic number’ instead of ‘atomic mass’. Forgetting to mention the gaps left for undiscovered elements. Saying ‘different elements have different masses’ instead of linking it to isotopes.

Mark scheme (4 marks)

  1. Mendeleev arranged elements in order of atomic weights / increasing atomic mass
  2. Elements with similar chemical properties were placed in the same group / column
  3. Mendeleev left gaps where elements had not yet been discovered / found
  4. His ordering was not always correct because different isotopes of an element have different masses / atomic weights

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