Revision notes for AQA GCSE Chemistry Development of the periodic table. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Development of the periodic table
What you'll learn
- Why early scientists first arranged elements using atomic weight.
- Why some early periodic tables had wrong-looking groups and missing elements.
- How Mendeleev improved the table by leaving gaps and making predictions.
- How isotopes and atomic number explain why the modern table is ordered differently.
The big idea
The periodic table did not appear perfectly formed. It developed as scientists collected more evidence, tested patterns, and improved their explanations.
A periodic table is an arrangement of elements designed to show repeating patterns in their properties. A property is a characteristic of a substance, such as how reactive it is or what compounds it forms.
Before the modern atom was understood
Today, you learn about atoms containing protons, neutrons and electrons. Early chemists did not know about these particles, so they could not arrange elements by atomic number.
Definition
Element
An element is a substance made from only one type of atom. Each element has its own chemical symbol, such as H for hydrogen, O for oxygen, or Fe for iron.
Early scientists could still measure masses and observe reactions. For example, they could compare how elements reacted with oxygen, water, acids or chlorine. These observations helped them spot patterns.
Definition
Atomic weight
Atomic weight was the historical name for the relative mass of atoms of an element. In GCSE chemistry, this is closely related to relative atomic mass, which is the average mass of the atoms of an element compared with carbon-12.
Early attempts: arranging by atomic weight
A sensible first idea was: put the known elements in order of increasing atomic weight.
This did reveal some patterns. Elements with similar chemical properties often appeared at regular intervals. When something happens in a repeating pattern, it is called periodic.
The modern periodic table has:
- Groups: vertical columns. Elements in the same group have similar chemical properties.
- Periods: horizontal rows. Moving across a period, properties change in a pattern.
Key Idea
Periodic means repeating
The periodic table is built around repeating patterns in chemical properties. The order matters because it decides which elements line up in the same groups.
The problems with strict atomic-weight order
Early periodic tables were useful, but they were not perfect.
Problem 1: the tables were incomplete
Many elements had not yet been discovered. If a scientist tried to fit every known element into a table with no gaps, the pattern could be distorted.
Think of it like trying to complete a puzzle when some pieces are still missing: if you force the pieces together, the picture may look wrong.
Problem 2: some elements ended up in inappropriate groups
If scientists followed atomic weight order too strictly, some elements were placed in groups with elements that did not behave like them.
That was a serious problem, because elements in the same group should have similar chemical properties.
Common Mistake
Atomic weight is not atomic number
Do not write that early scientists arranged elements by atomic number. They could not do that because protons had not been discovered. Early tables were mainly based on atomic weight and chemical properties.
Mendeleev’s improvement
Dmitri Mendeleev was a Russian chemist who produced a much more successful periodic table in 1869.
Mendeleev mostly arranged elements in order of increasing atomic weight, but he did two clever things.
1. He left gaps
Mendeleev left gaps where he thought undiscovered elements should go. This helped preserve the pattern of similar properties in groups.
He did not treat the gaps as mistakes. Instead, he used the surrounding elements to predict the properties of the missing elements.
Definition
Prediction
A prediction is a statement about what should happen or what should be found if a scientific idea is correct.
2. He changed the order in some places
Mendeleev sometimes swapped the order of elements so that elements with similar chemical properties stayed in the same group.
This meant he did not follow atomic weight order blindly. He trusted the chemical pattern when there was strong evidence from the elements’ behaviour.
Example
Choosing the order of tellurium and iodine
Suppose two elements have these facts: iodine has atomic weight 126.90 and reacts like chlorine and bromine; tellurium has atomic weight 127.60 and reacts like sulfur and selenium.
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Strict atomic-weight order would put iodine before tellurium, because 126.90 is less than 127.60.
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The chemical-property pattern says tellurium belongs with sulfur and selenium, while iodine belongs with chlorine and bromine.
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Mendeleev would prioritise the repeating property pattern, so he would place tellurium before iodine even though this reverses strict atomic-weight order.
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Modern atomic numbers support that choice: tellurium has Z=52Z = 52Z=52 and iodine has Z=53Z = 53Z=53, so increasing atomic number places tellurium before iodine.
Why Mendeleev’s predictions mattered
Mendeleev’s table became accepted because some of his predicted elements were later discovered, and their properties matched his predictions closely.
This is important for Working Scientifically: a scientific idea becomes stronger when it makes predictions that can be tested. If the predictions match new evidence, the idea is supported. If they do not match, the idea may need changing or rejecting.
Example
Testing Mendeleev’s prediction
Mendeleev predicted that a missing element below aluminium would have an atomic weight of about 68 and would form compounds similar to aluminium compounds. Gallium was later discovered with relative atomic mass about 69.7 and similar chemical behaviour.
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Compare the predicted mass with the measured mass: about 68 is close to 69.7, so the numerical evidence supports the prediction.
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Compare the predicted chemical behaviour with the observed behaviour: gallium forms compounds similar to aluminium, so the chemical evidence also supports the prediction.
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Combine the evidence: both mass and properties fit the gap in Mendeleev’s table, so the discovery of gallium strengthened confidence in his arrangement.
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Consider the alternative: if gallium had very different properties, it would have weakened Mendeleev’s idea.
Key Idea
Why the gaps were powerful
Mendeleev’s gaps were not just empty spaces. They were testable predictions about elements that had not yet been discovered.
The modern explanation: atomic number
Later, scientists discovered that atoms contain smaller particles.
- Protons are positively charged particles in the nucleus of an atom.
- Neutrons are neutral particles in the nucleus.
- Electrons are negatively charged particles found around the nucleus.
The nucleus is the tiny central part of an atom containing protons and neutrons.
Definition
Atomic number
The atomic number, often written as ZZZ, is the number of protons in the nucleus of an atom.
The modern periodic table is arranged in order of increasing atomic number, not increasing atomic weight.
This works because the number of protons determines which element an atom is. In a neutral atom, the number of electrons is the same as the number of protons, and the electron arrangement affects chemical properties.
So atomic number gives a better underlying explanation for the repeating patterns in the periodic table.
Why isotopes explained the old anomalies
Some atomic-weight order problems were explained by the discovery of isotopes.
Definition
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Because isotopes have different numbers of neutrons, they have different masses. The atomic weight of an element is an average of the masses of its isotopes.
That means an element can have a slightly higher average atomic weight than the next element, even though it has a lower atomic number.
This explains cases like tellurium and iodine:
- Tellurium has atomic number 52.
- Iodine has atomic number 53.
- Tellurium’s average atomic weight is slightly higher than iodine’s because of its isotopes.
- The modern order still places tellurium before iodine because atomic number is the correct basis.
Tip
A strong explanation chain
For this topic, a good answer often follows this chain: atomic weight order → problems with groups/gaps → Mendeleev’s gaps and swaps → predictions confirmed → isotopes and atomic number explain the modern order.
What you need to be able to describe
For GCSE Chemistry, you should be able to describe the development as a sequence:
- Before protons, neutrons and electrons were discovered, scientists arranged elements by atomic weight.
- Early tables were incomplete because some elements had not been discovered.
- Strict atomic-weight order sometimes put elements in groups where they did not match the chemical properties of the other elements.
- Mendeleev improved the table by leaving gaps for undiscovered elements.
- Mendeleev predicted the properties of missing elements.
- When elements were discovered that matched his predictions, his table gained support.
- The discovery of isotopes explained why atomic weight order was not always correct.
- The modern periodic table is arranged by atomic number.
Exam technique
In the exam
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If asked about early periodic tables, mention atomic weight and the problem of inappropriate groups or missing elements.
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If asked about Mendeleev, include both key improvements: he left gaps and he sometimes changed the order to keep similar properties together.
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If asked why Mendeleev’s table was accepted, explain that later discoveries matched his predictions, which supported his scientific idea.
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If asked about the modern explanation, say that isotopes affect atomic weight, but atomic number depends on protons and gives the correct order.
Self review
Check yourself
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Why could early scientists not arrange elements by atomic number?
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What two things did Mendeleev do that made his periodic table more successful?
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How do isotopes explain why atomic-weight order was not always correct?
The periodic table
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