AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Mendeleev overcame some of the problems by leaving gaps for elements that…
Full specification wording
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.
Mendeleev improved on earlier tables by prioritising chemical properties over strict atomic-weight order.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Mendeleev improved on earlier tables by prioritising chemical properties over strict atomic-weight order. Where the next atomic weight would have placed an element in a group that did not match its chemistry, he swapped the order. He also left deliberate gaps rather than forcing every known element into a continuous sequence. For each gap he predicted the properties of the missing element from the properties of its neighbours. This approach turned the periodic table into a predictive model: it not only organised known elements but also made testable forecasts. For example, he predicted the existence and properties of an element he called eka-silicon, which was later discovered as germanium. His willingness to deviate from strict atomic-weight order was a key strength of his method.
What earns the marks
- State that Mendeleev left gaps in his table for elements he believed were undiscovered.
- Explain that he sometimes reversed the strict atomic-weight order to keep elements with similar properties in the same group.
- Describe how he used the properties of neighbouring elements to predict the properties of missing elements.
- Give a named example of a predicted element, such as eka-silicon, later identified as germanium.
- Recognise that Mendeleev's table was more successful because it prioritised chemical properties and made testable predictions.
Common mistakes and how to put them right
Saying Mendeleev left gaps because he ran out of space.
Put it right by explaining the predictive purpose.
Claiming Mendeleev ignored atomic weight completely.
Put it right by describing his selective reordering.
Confusing Mendeleev's predictions with guesses.
Put it right by linking predictions to periodic trends.
Exam tips
- Explain the reason for each gap rather than just stating that gaps existed.
- Use one named prediction and its later discovery to show how Mendeleev's table was tested.
- When describing changes to order, say that chemical properties took priority over strict atomic weight.
Try the lesson
The start of the 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. lesson, made from checked teaching notes for this specification point.
Have a go first
What did Mendeleev do to overcome problems with the early periodic tables?
Listen
Mendeleev improved on earlier tables by prioritising chemical properties over strict atomic-weight order. Where the next atomic weight would have placed an element in a group that did not match its chemistry, he swapped the order. He also left deliberate gaps rather than forcing every known element into a continuous sequence. For each gap he predicted the properties of the missing element from the properties of its neighbours. This approach turned the periodic table into a predictive model: it not only organised known elements but also made testable forecasts. For example, he predicted the existence and properties of an element he called eka-silicon, which was later discovered as germanium. His willingness to deviate from strict atomic-weight order was a key strength of his method. A strong answer earns marks for these points. State that Mendeleev left gaps in his table for elements he believed were undiscovered. Explain that he sometimes reversed the strict atomic-weight order to keep elements with similar properties in the same group. Describe how he used the properties of neighbouring elements to predict the properties of missing elements. Give a named example of a predicted element, such as eka-silicon, later identified as germanium. A common slip: Saying Mendeleev left gaps because he ran out of space; the gaps were deliberate predictions of undiscovered elements. Instead, explaining the predictive purpose. Exam tip: Explain the reason for each gap rather than just stating that gaps existed.
See it
Key words
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Predictive power of scientific models
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Prioritising chemical properties over atomic weight
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Role of gaps in classification
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Testing predictions against new evidence
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See every definition
- Predictive power of scientific models
- The approach turned the periodic table into a predictive model that not only organised known elements but also made testable forecasts.
- Prioritising chemical properties over atomic weight
- Where the next atomic weight would have placed an element in a group that did not match its chemistry, he swapped the order.
- Role of gaps in classification
- He left deliberate gaps rather than forcing every known element into a continuous sequence, and for each gap predicted the properties of the missing element from its neighbours.
- Testing predictions against new evidence
- He predicted the existence and properties of an element he called eka-silicon, which was later discovered as germanium.
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