Mendeleev arranged elements into a periodic table in 1869. His table was different from the modern periodic table. Explain how Mendeleev arranged elements in his table and describe two ways in which the modern periodic table differs from Mendeleev's original table.

Edexcel A-Level Chemistry (9CH0) — 1.2 Periodic table and trends · Explain · 5 marks · View as Markdown

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

When Mendeleev published his periodic table in 1869, not all elements had been discovered. He made predictions about undiscovered elements such as eka-silicon, which was later found to be the element germanium.

Model answer (5 marks)

Mendeleev arranged the elements in order of increasing relative atomic mass, grouping those with similar chemical properties together. He deliberately left empty spaces in the table where he expected new elements to be found, and he even predicted the properties of these missing elements, such as eka‑silicon (later germanium).

The modern periodic table differs in two main ways:
1. It is arranged by increasing atomic number (the number of protons) rather than relative atomic mass, which removes the ordering errors that appeared in Mendeleev’s table.
2. It contains all currently known elements, including the noble gases (Group 18), which were absent from Mendeleev’s original table; a new group has therefore been added to accommodate them.

Examiner tips

  • Use the word "arranged" and mention "relative atomic mass" for the first part. Show the two differences clearly: atomic number and inclusion of noble gases. Keep the answer concise to fit the 5 marks.

Common mistakes

  • Confusing atomic mass with atomic number when describing Mendeleev’s arrangement. Forgetting to mention the noble gases or the new group added in the modern table.

Mark scheme (5 marks)

  1. Mendeleev arranged elements in order of relative atomic mass (atomic weight)
  2. Mendeleev left gaps in his table for undiscovered elements / predicted the existence and properties of undiscovered elements
  3. The modern periodic table is arranged in order of atomic number (proton number) rather than relative atomic mass
  4. The modern periodic table includes more elements / all currently known elements have been discovered and placed
  5. The noble gases (Group 0) are included in the modern periodic table but were not in Mendeleev's table / a new group was added

Key terms in this question

periodic table

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