Development of the Periodic Table
Master Development of the Periodic Table for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Mendeleev left gaps and even reordered some elements to fit their properties, correctly predicting elements not yet discovered.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The modern periodic table arranges elements in order of increasing atomic number, in rows called periods and columns called groups.
Early attempts arranged elements by atomic weight, which placed some elements in inappropriate groups because isotopes had not been discovered. Mendeleev overcame this by leaving gaps for elements not yet discovered and by occasionally changing the order.
The properties of the missing elements he predicted were later confirmed when those elements were discovered, which is what established his table. Once isotopes were discovered, the apparent problems with atomic weight order were explained.
Revision notes
Early tables
Elements were originally arranged in order of atomic weight.
This placed some elements in groups with elements having very different properties, because isotopes had not yet been discovered and atomic weights are averages.
Mendeleev's approach
Mendeleev left gaps for elements he believed had not yet been discovered.
He also changed the order of some elements where their properties demanded it. He predicted the properties of the missing elements, and these predictions were confirmed when the elements were found.
The modern table
Elements are arranged in order of increasing atomic number.
Elements with similar properties fall into the same column, called a group. Rows are called periods. Knowledge of isotopes explained why atomic weight order sometimes appeared wrong.
Key points
- The modern table is ordered by atomic number.
- Early tables used atomic weight.
- Atomic weight order placed some elements incorrectly.
- Mendeleev left gaps for undiscovered elements.
- His predictions were later confirmed.
- Isotopes explained the atomic weight problem.
Worked examples
Example 1
Explain why Mendeleev left gaps in his periodic table. [2 marks]
Model answer
He believed there were elements that had not yet been discovered (1 mark), and leaving gaps allowed him to keep elements with similar properties in the same group (1 mark).
Example 2
Explain why arranging elements by atomic weight caused problems. [2 marks]
Model answer
Isotopes had not been discovered, so atomic weights are averages that do not always increase in the same order as atomic number (1 mark). This placed some elements in groups with elements having very different properties (1 mark).
Example 3
State how the modern periodic table is arranged. [2 marks]
Model answer
Elements are arranged in order of increasing atomic number (1 mark), with elements having similar properties placed in the same group or column (1 mark).
Common mistakes
Saying the modern table uses atomic weight.
It uses atomic number. Atomic weight was the early, problematic approach.
Saying Mendeleev knew about isotopes.
They had not been discovered; that explanation came later.
Forgetting that his predictions were confirmed.
That confirmation is what established his table as correct.
Confusing groups and periods.
Groups are columns; periods are rows.
Exam tips
- Say atomic number for the modern table.
- Explain the atomic weight problem through isotopes.
- Mention that Mendeleev's predictions were confirmed.
- Use this topic as an example of scientific prediction and evidence.
Key terms
- Periodic table
- An arrangement of elements by increasing atomic number.
- Group
- A column of elements with similar properties.
- Period
- A row of the periodic table.
- Mendeleev
- The scientist who left gaps for undiscovered elements.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.