AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The early periodic tables were incomplete and some elements were placed in…
Full specification wording
Development of the periodic table: The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.
Early periodic tables were built from the elements known at the time, so many gaps existed for undiscovered elements.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Early periodic tables were built from the elements known at the time, so many gaps existed for undiscovered elements. When chemists forced every element into strict atomic-weight order, some elements landed in groups where their chemical properties did not match those of their neighbours. Tellurium and iodine provide a classic problem: tellurium has the greater atomic weight but behaves more like the elements in the group before iodine. Argon and potassium present a similar issue. Such mismatches showed that atomic weight was not the fundamental property controlling chemical behaviour. The tables were therefore incomplete in two senses: missing elements and misplaced elements.
What earns the marks
- State that early tables contained gaps because many elements had not yet been discovered.
- Explain that strict atomic-weight ordering sometimes placed elements in groups that did not match their chemical properties.
- Give a specific example of a pair (such as tellurium and iodine) whose position conflicted with its properties under strict atomic-weight ordering.
- Recognise that these problems showed atomic weight was not the fundamental property determining chemical behaviour.
- Describe the consequence: early tables were provisional and required revision as new evidence emerged.
Common mistakes and how to put them right
Claiming that early tables were complete but simply wrong.
Put it right by mentioning the gaps.
Saying that misplaced elements were placed randomly.
Put it right by linking the misplacement to the ordering rule.
Confusing a gap with a misplaced element.
Put it right by defining each term clearly.
Exam tips
- Use the phrase strict atomic-weight order when explaining why some elements ended up in inappropriate groups.
- Name one pair (e.g. tellurium and iodine) to support your explanation; a specific example is more convincing than a general statement.
- Distinguish clearly between an incomplete table and an incorrect placement when answering questions.
Try the lesson
The start of the Development of the periodic table: The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed. lesson, made from checked teaching notes for this specification point.
Have a go first
What was one problem with the early periodic tables when the strict order of atomic weights was followed?
Listen
Early periodic tables were built from the elements known at the time, so many gaps existed for undiscovered elements. When chemists forced every element into strict atomic-weight order, some elements landed in groups where their chemical properties did not match those of their neighbours. Tellurium and iodine provide a classic problem: tellurium has the greater atomic weight but behaves more like the elements in the group before iodine. Argon and potassium present a similar issue. Such mismatches showed that atomic weight was not the fundamental property controlling chemical behaviour. The tables were therefore incomplete in two senses: missing elements and misplaced elements. A strong answer earns marks for these points. State that early tables contained gaps because many elements had not yet been discovered. Explain that strict atomic-weight ordering sometimes placed elements in groups that did not match their chemical properties. Give a specific example of a pair (such as tellurium and iodine) whose position conflicted with its properties under strict atomic-weight ordering. Recognise that these problems showed atomic weight was not the fundamental property determining chemical behaviour. A common slip: Claiming that early tables were complete but simply wrong; they were incomplete because many elements were unknown. Instead, mentioning the gaps. Exam tip: Use the phrase strict atomic-weight order when explaining why some elements ended up in inappropriate groups.
Key words
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Limitations of early classification systems
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Incompleteness of scientific evidence
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Conflict between ordering principle and chemical properties
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Provisional nature of scientific models
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See every definition
- Limitations of early classification systems
- Early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.
- Incompleteness of scientific evidence
- Early tables contained gaps because many elements had not yet been discovered, so they were incomplete in the sense of missing elements.
- Conflict between ordering principle and chemical properties
- Strict atomic-weight ordering sometimes placed elements in groups that did not match their chemical properties, as with tellurium and iodine.
- Provisional nature of scientific models
- Such mismatches showed that atomic weight was not the fundamental property controlling chemical behaviour, so early tables were provisional and required revision as new evidence emerged.
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