Development of the Periodic Table — AQA GCSE Chemistry
The modern periodic table is the result of scientists organising the elements and refining their ideas over time.
Early attempts
Before the structure of the atom was understood, scientists tried to organise the elements by their atomic weights and properties. Early tables were incomplete and sometimes placed elements in the wrong groups.
Mendeleev's breakthrough
Dmitri Mendeleev (1869) arranged the known elements in order of atomic weight, but he:
- left gaps for elements that had not yet been discovered, and
- predicted the properties of these missing elements.
When elements like gallium and germanium were later discovered with properties matching his predictions, his table was accepted. He also switched the order of some elements so they fitted the patterns of properties.
The modern table
- The discovery of isotopes explained why ordering by atomic weight sometimes failed.
- The modern table is arranged in order of atomic (proton) number, which removes the problems Mendeleev faced.
- Elements with similar properties are placed in the same group (column), reflecting their electronic structure.
Metals and non-metals
The table is divided into metals (left and centre) and non-metals (right). Metals form positive ions; non-metals tend to gain or share electrons.
Exam tips
- Mendeleev's key ideas: ordered by atomic weight, left gaps, predicted properties.
- The modern table is ordered by atomic number.
- The discovery of isotopes explained the anomalies in weight ordering.
- Same group = similar properties = same number of outer electrons.