AQA · Chemistry · 8462
GCSE Chemistry: Development of the periodic table
Atomic structure and the periodic table · AQA GCSE Chemistry (8462), specification point 4.1.2.2. Cards are generated from the exam board's own specification for "Development of the periodic table", and every card links back to the page it came from. Specification © AQA — used with attribution.
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Before protons, neutrons and electrons were discovered, what property did scientists use to order the elements?
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Atomic weight — elements were arranged in order of increasing atomic weight.
Why were some elements placed in inappropriate groups in the early periodic tables?
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Because the strict order of atomic weights was followed, which sometimes put elements with dissimilar properties together.
Why did Mendeleev leave gaps in his periodic table?
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For elements he thought had not yet been discovered.
Besides leaving gaps, what other change did Mendeleev make to the strict atomic-weight order?
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In some places he swapped the order of elements so that they fell into groups with similar properties.
What evidence supported Mendeleev's periodic table after it was published?
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Elements with the properties he had predicted were discovered and filled the gaps.
What later discovery explained why ordering by atomic weight was not always correct?
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The discovery of isotopes — atoms of the same element with different masses affect the average atomic weight.
How can testing a prediction support a new scientific idea?
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If the prediction turns out to be correct (e.g. Mendeleev's predicted elements being discovered), it gives evidence that the idea is valid; if it fails, the idea is refuted.
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