Development of the periodic table: Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.

The idea

Mendeleev improved on earlier periodic tables by prioritising chemical and physical properties over strict atomic weight order. Where the mass order would have placed an element in a group whose properties did not fit, he swapped the order. He also left gaps in his table for elements he predicted were undiscovered, and used the gaps and trends to predict their properties. When gallium, scandium and germanium were later discovered, their properties matched his predictions closely, which gave strong support to his table. This shows how a model can be tested by prediction and subsequent evidence, and why Mendeleev's arrangement became the basis of the modern periodic table.

What earns the marks

• Mendeleev left gaps in his table for elements he believed were yet to be discovered.
• He sometimes reversed the strict atomic weight order so that elements with similar properties fell into the same group.
• He used the gaps and the trends in his table to predict the properties of undiscovered elements.
• Later discovery of elements such as gallium, scandium and germanium supported his predictions and the value of his table.
• His approach showed that chemical properties, not mass alone, were the better basis for arranging elements.

Common mistakes and how to put them right

• Saying Mendeleev left gaps because he had run out of space.

  Put it right: He left gaps deliberately for undiscovered elements and predicted their properties.

• Believing Mendeleev ignored atomic weights completely.

  Put it right: He used atomic weights but changed the order where properties demanded it.

• Confusing Mendeleev's predictions with the modern table's use of atomic number.

  Put it right: Mendeleev worked with atomic weights; atomic number came later.

Exam tips

• State clearly that Mendeleev changed the order in some places and left gaps, then explain why each action helped.
• Use one named predicted element, such as gallium or germanium, to show how later discoveries supported his table.
• Contrast Mendeleev's approach with earlier strict mass-order tables to show the improvement.