Mendeleev arranged the elements in an early version of the periodic table. Describe how Mendeleev arranged the elements in his periodic table and explain why his arrangement was significant.

Eduqas A-Level Chemistry — 2.1 Periodic table · Describe and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Mendeleev arranged the elements in order of increasing atomic weight.
He grouped elements with similar chemical properties into the same vertical column.
He deliberately left empty spaces where no known element fit the pattern.
These gaps enabled him to predict the properties of yet‑unknown elements.
When those elements were later discovered and matched his predictions, his table gained acceptance as a valid model.

Examiner tips

  • Use the word "arranged" and mention atomic weight first. Show that groups contain similar properties. Explain the purpose of the gaps and the predictive success.
  • Remember to link the predictive success to the acceptance of the table.

Common mistakes

  • Confusing atomic number with atomic weight. Failing to mention that groups contain similar properties. Not explaining why the gaps were significant.

Mark scheme (5 marks)

  1. Mendeleev arranged the elements in order of increasing atomic weight (atomic mass)
  2. Elements with similar chemical properties were placed in the same group (column)
  3. Mendeleev left gaps in his table where no known element fitted the pattern
  4. The gaps allowed Mendeleev to predict the properties of elements that had not yet been discovered
  5. When those elements were later discovered and matched his predictions, his table was accepted / this showed the periodic table was a valid model

Key terms in this question

periodic table · period

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