AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
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.
Before Mendeleev, chemists arranged elements by atomic weight, but their tables had two linked weaknesses.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Before Mendeleev, chemists arranged elements by atomic weight, but their tables had two linked weaknesses. First, they were incomplete: only known elements could be included, leaving no gaps for undiscovered ones. Second, forcing every element into strict atomic weight order put some elements into groups where their properties did not match. For example, iodine (Ar ≈ 127) is lighter than tellurium (Ar ≈ 128). Strict weight order places iodine before tellurium, but iodine's properties match chlorine and bromine, while tellurium resembles sulfur and selenium. Early tables broke down because mass order and chemical behaviour disagreed.
What earns the marks
- Early tables were built using atomic weights rather than atomic number.
- The tables were incomplete because only known elements could be placed, leaving no gaps for undiscovered elements.
- Following strict atomic weight order sometimes placed an element in a group with different chemical properties.
- The mismatch between mass order and property patterns showed that atomic weight alone was not fully reliable.
- A named example such as iodine and tellurium illustrates how strict mass order places iodine before tellurium, conflicting with observed group similarity.
Common mistakes and how to put them right
Stating strict mass order places iodine after tellurium.
Put it right: Iodine is lighter, so strict mass order places it before tellurium.
Claiming the tables were incomplete only because scientists had not tried hard enough.
Put it right: Explain that undiscovered elements were absent.
Treating every early table as identical.
Put it right: Recognise that different chemists proposed different arrangements, but all shared the mass-order limitation.
Exam tips
- Distinguish between the two problems: incompleteness was due to undiscovered elements, while inappropriate grouping stemmed from strictly following atomic weight order.
- Use the iodine and tellurium example to show how strict mass order places iodine before tellurium, conflicting with chemical similarity.
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
Why were some elements placed in inappropriate groups in early periodic tables?
Listen
Before Mendeleev, chemists arranged elements by atomic weight, but their tables had two linked weaknesses. First, they were incomplete: only known elements could be included, leaving no gaps for undiscovered ones. Second, forcing every element into strict atomic weight order put some elements into groups where their properties did not match. For example, iodine (Ar ≈ 127) is lighter than tellurium (Ar ≈ 128). Strict weight order places iodine before tellurium, but iodine's properties match chlorine and bromine, while tellurium resembles sulfur and selenium. Early tables broke down because mass order and chemical behaviour disagreed. A strong answer earns marks for these points. Early tables were built using atomic weights rather than atomic number. The tables were incomplete because only known elements could be placed, leaving no gaps for undiscovered elements. Following strict atomic weight order sometimes placed an element in a group with different chemical properties. The mismatch between mass order and property patterns showed that atomic weight alone was not fully reliable. A common slip: Stating strict mass order places iodine after tellurium. Instead, iodine is lighter, so strict mass order places it before tellurium. Exam tip: Distinguish between the two problems: incompleteness was due to undiscovered elements, while inappropriate grouping stemmed from strictly following atomic weight order.
Key words
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Atomic weight
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Incomplete
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Inappropriate groups
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Patterns in the periodic table
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See every definition
- Atomic weight
- The quantity by which early periodic tables arranged elements, though following its strict order could place elements in inappropriate groups.
- Incomplete
- Describes early periodic tables that could only include known elements, leaving no gaps for undiscovered ones.
- Inappropriate groups
- Groups in which some elements were placed if the strict order of atomic weights was followed, where their properties did not match.
- Patterns in the periodic table
- The property patterns that showed atomic weight alone was not fully reliable when mass order and chemical behaviour disagreed.
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