Development of the periodic table: Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.

The idea

Mendeleev arranged elements mainly by increasing atomic weight but left gaps where no element fitted the pattern. He predicted properties of the missing elements, such as a metal he called eka-silicon, and when gallium and germanium were later discovered their properties matched his predictions, so his table gained acceptance. However, a few pairs of elements appeared in the wrong order when arranged strictly by atomic weight, for example tellurium before iodine. After isotopes were discovered, scientists understood that atoms of the same element can have different masses, so atomic weight is an average and is not the fundamental ordering property. Ordering by atomic (proton) number removes the anomalies and explains the pattern.

What earns the marks

• Mendeleev arranged elements mainly in order of increasing atomic weight and left gaps for elements not yet discovered.
• He predicted the properties of missing elements from the properties of their neighbours, and later discoveries such as gallium and germanium matched those predictions.
• The success of these predictions supported Mendeleev's table and helped it replace earlier arrangements.
• Some pairs of elements, such as tellurium and iodine, were placed out of strict atomic-weight order because their chemical properties fitted better that way.
• Isotopes are atoms of the same element with different masses, so relative atomic mass is an average and does not uniquely determine an element's position.
• Ordering by atomic (proton) number explains the periodic pattern and removes the anomalies seen with atomic weight.

Common mistakes and how to put them right

• Saying Mendeleev arranged elements by atomic number.

  Put it right by stating he used atomic weight, and that atomic number was introduced later.

• Claiming Mendeleev predicted the discovery of elements.

  Put it right by saying he predicted their properties, and the elements were discovered later.

• Stating that isotopes have different chemical properties.

  Put it right by explaining that isotopes of an element have the same proton number and so similar chemical properties, but different masses.

Exam tips

• When describing Mendeleev's table, always mention both the gaps and the predictions, because these are the key evidence that made his table successful.
• Use a named example such as gallium or germanium to show that predicted properties were confirmed.
• If asked why atomic weight caused problems, refer to isotopes and to the fact that relative atomic mass is an average value.