Development of the periodic table (AQA GCSE Chemistry Combined Science): Revision Notes
Development of the periodic table
Early attempts to classify elements
In the 19th century, many elements had been discovered. Scientists tried to organise them in a useful way. Mendeleev was one scientist who made an important breakthrough.
The 19th century was a period of rapid scientific discovery, with many new elements being identified. Scientists needed a systematic way to understand and organise this growing collection of elements.
What scientists knew in the 1800s
Information available
Scientists in the 19th century knew about:
- Properties of elements - how they behaved and reacted
- Atomic weights - how heavy the atoms were compared to each other
What they didn't know
Scientists had no idea about:
- Protons, neutrons and electrons - the particles inside atoms
- Atomic numbers - how many protons each element had
Atomic weight is like relative atomic mass. But many atomic weights calculated back then were wrong.
First attempts at a periodic table
The early table problems
The first periodic table tried to organise elements by putting them in order of atomic weight. It had seven columns.
Problems with this approach:
- After calcium, the pattern completely broke down
- Metals and non-metals ended up mixed together in the same columns
- For example, oxygen and iron were in the same column - but they're completely different!
- There was no space left for new elements that might be discovered
Mendeleev's breakthrough
What made Mendeleev different
Mendeleev published his periodic table in 1869. His clever ideas were:
- He left gaps in his table where he thought undiscovered elements should go
- He swapped some elements around - if their properties didn't fit, he put them where they belonged based on behaviour, not just atomic weight
- He made predictions about what the missing elements would be like
Example of swapping elements: Iodine and Tellurium
Iodine has a lower atomic weight than tellurium. Following strict atomic weight order would put iodine first. But Mendeleev swapped them around because iodine's properties fitted better with the other elements in its group.
This showed that properties matter more than just atomic weight when organising elements.
Why this worked
Mendeleev realised that properties repeat in a pattern. Elements with similar properties appeared at regular intervals. This is why we call it the periodic table - the pattern repeats periodically.
Mendeleev's predictions came true
The most amazing thing about Mendeleev's table was that he predicted properties of elements that hadn't been discovered yet. When these elements were later found, their properties matched his predictions almost perfectly!
This proved that Mendeleev's approach was correct. He had found the real pattern that organises the elements.
Why the modern periodic table works
Today's periodic table has over 100 elements. It's called the periodic table because elements with similar properties appear at regular intervals throughout the table.
Key Points to Remember:
- Mendeleev organised elements by properties, not just atomic weight
- He left gaps for undiscovered elements and made accurate predictions
- He sometimes swapped elements to make the pattern of properties work better
- His predictions for missing elements turned out to be correct when they were discovered
- The periodic table works because properties of elements repeat in a regular pattern