AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Before the discovery of protons, neutrons and electrons, scientists…
Full specification wording
Development of the periodic table: Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.
Long before sub-atomic particles were known, chemists had only measurable properties such as relative atomic mass to work with.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 6
Study notes
From the checked teaching notes for this specification point.
The idea
Long before sub-atomic particles were known, chemists had only measurable properties such as relative atomic mass to work with. They noticed that certain elements resembled each other chemically and tried to organise everything into a single sequence. The simplest method was to list elements in order of increasing atomic weight, hoping that similar elements would fall at regular intervals. Early attempts, such as Newlands' octaves, did show repeating patterns, but the sequences broke down because atomic weight does not always increase in step with chemical behaviour. For example, argon has a greater atomic weight than potassium yet sits before it in the modern table. Without knowledge of protons, neutrons and electrons, scientists could not explain why the order sometimes failed, so classification remained a useful but imperfect tool.
What earns the marks
- State that early classification used atomic weight as the only numerical ordering principle available.
- Explain that atomic weight was chosen because protons, neutrons and electrons had not yet been discovered.
- Describe the aim of early tables: to group elements with similar chemical properties together.
- Recognise that ordering by atomic weight produced repeating patterns for some groups of elements but not all.
- Give a named example of an early attempt, such as Newlands' octaves, and note its limitation.
Common mistakes and how to put them right
Writing that early scientists arranged elements by atomic number.
Put it right by saying they used atomic weight.
Assuming all early tables were complete and correct.
Put it right by describing them as provisional classifications.
Confusing atomic weight with mass number.
Put it right by using the term relative atomic mass.
Exam tips
- Link the method of ordering directly to the limited evidence available before sub-atomic particles were discovered.
- Use one named early scientist or table to show secure historical knowledge rather than a vague generalisation.
- When comparing early and modern tables, state the ordering principle used in each rather than just saying one is better.
Try the lesson
The start of the Development of the periodic table: Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights. lesson, made from checked teaching notes for this specification point.
Have a go first
Before protons, neutrons and electrons were discovered, in what order did scientists arrange the elements when they tried to classify them?
Listen
Long before sub-atomic particles were known, chemists had only measurable properties such as relative atomic mass to work with. They noticed that certain elements resembled each other chemically and tried to organise everything into a single sequence. The simplest method was to list elements in order of increasing atomic weight, hoping that similar elements would fall at regular intervals. Early attempts, such as Newlands' octaves, did show repeating patterns, but the sequences broke down because atomic weight does not always increase in step with chemical behaviour. For example, argon has a greater atomic weight than potassium yet sits before it in the modern table. Without knowledge of protons, neutrons and electrons, scientists could not explain why the order sometimes failed, so classification remained a useful but imperfect tool. A strong answer earns marks for these points. State that early classification used atomic weight as the only numerical ordering principle available. Explain that atomic weight was chosen because protons, neutrons and electrons had not yet been discovered. Describe the aim of early tables: to group elements with similar chemical properties together. Recognise that ordering by atomic weight produced repeating patterns for some groups of elements but not all. A common slip: Writing that early scientists arranged elements by atomic number; atomic number depends on proton number, which was unknown at the time. Instead, saying they used atomic weight. Exam tip: Link the method of ordering directly to the limited evidence available before sub-atomic particles were discovered.
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