Development of the periodic table: Students should be able to describe these steps in the development of the periodic table.

The idea

To describe the steps in the development of the periodic table, start with the early grouping of elements by similar properties, then describe Newlands' law of octaves, which arranged elements by atomic weight but was not accepted because the pattern broke down for heavier elements. Then describe Mendeleev's table, arranged mainly by atomic weight with gaps left for undiscovered elements and with properties predicted for those gaps. Later, the discovery of gallium and germanium matched his predictions, and the discovery of isotopes explained why a strict atomic-weight order sometimes placed elements incorrectly. Finally, the modern table orders elements by atomic (proton) number, which explains the pattern without anomalies.

What earns the marks

• Early attempts grouped elements by similar chemical and physical properties.
• Newlands arranged elements in order of atomic weight and proposed a repeating pattern, but his ideas were not widely accepted because the pattern did not hold for all elements.
• Mendeleev arranged elements mainly by atomic weight, left gaps for undiscovered elements and predicted their properties.
• The later discovery of elements such as gallium and germanium, with properties close to Mendeleev's predictions, supported his table.
• Isotopes were discovered later, showing that atomic weight is an average and explaining why strict atomic-weight ordering sometimes failed.
• The modern periodic table orders elements by atomic (proton) number, which removes the anomalies and explains the repeating pattern.

Common mistakes and how to put them right

• Describing the steps in the wrong order.

  Put it right by sequencing early property grouping, Newlands, Mendeleev, isotopes and the modern atomic-number table.

• Saying Newlands' table was accepted immediately.

  Put it right by stating that it was criticised and not widely accepted at the time.

• Confusing atomic weight with atomic number.

  Put it right by stating that Mendeleev used atomic weight, while the modern table uses atomic number.

Exam tips

• Use a clear sequence with connectives such as first, then, later and finally to show the order of the steps.
• Include one named element whose discovery supported Mendeleev's predictions, such as gallium or germanium.
• Link the isotope explanation to the modern table by stating that atomic number, not atomic weight, gives the correct order.