Development of the periodic table (AQA GCSE Chemistry): Revision Notes
Development of the periodic table
Early scientific knowledge
In the 19th century, scientists understood quite a lot about elements, but they were missing some crucial information.
What scientists knew:
- The properties of different elements
- The atomic weight of elements (similar to relative atomic mass)
What scientists did not know:
- About protons, neutrons and electrons
- About atomic numbers (proton numbers)
This missing knowledge about the internal structure of atoms made it difficult to organise elements properly. Scientists had to rely on observable properties and atomic weights alone.
First attempts at organising elements
Early scientists tried to create a periodic table by arranging elements in order of their atomic weight. Their first table had seven columns.
Problems with this early table:
- The pattern broke down after calcium
- Some metals and non-metals ended up in the same column (like oxygen and iron)
- There was no space left for new elements that might be discovered
Every seventh element seemed to have similar properties, but this pattern didn't work throughout the whole table.
The main flaw was that organising purely by atomic weight didn't account for undiscovered elements, causing the pattern to fail and creating confusion about element groupings.
Mendeleev's breakthrough
Dmitri Mendeleev published his periodic table in 1869. He made some clever changes that solved the problems of earlier tables.
Mendeleev's key ideas:
- He left gaps in his table where he thought undiscovered elements belonged
- He swapped some elements around to make sure elements with similar properties lined up in groups
- For example, he put iodine before tellurium even though iodine has a higher atomic mass
Example: The Iodine-Tellurium Swap
Mendeleev placed iodine (atomic mass 127) before tellurium (atomic mass 128) in his table, even though this went against the atomic weight order. He did this because:
- Iodine's properties matched better with the halogen group
- Tellurium's properties fit better with the group it was placed in
- This decision was later proven correct when atomic numbers were discovered
Why Mendeleev's approach worked
Mendeleev realised that some elements had not been discovered yet. By leaving spaces, he made sure that elements with similar properties appeared in the same columns (groups).
His brilliant prediction system:
- He predicted the properties of elements that would fill the gaps
- When these elements were later discovered, their properties matched his predictions almost exactly
- This proved his table was correct
The modern periodic table contains over 100 elements, but far fewer were known in 1869. It's called the "periodic" table because elements with similar properties appear at regular intervals.
Mendeleev's success in predicting the properties of undiscovered elements like gallium, scandium, and germanium provided strong evidence that his organisational method was fundamentally correct.
Key Points to Remember:
- Early scientists organised elements by atomic weight but didn't know about atomic numbers
- The first periodic tables had problems - patterns broke down and there was no room for new elements
- Mendeleev left gaps for undiscovered elements and swapped some elements to match their properties
- Mendeleev made accurate predictions about missing elements that were later proved correct
- This success showed that Mendeleev's method was the right approach to organising elements