Early attempts
As more elements were discovered, chemists searched for patterns. Johann Döbereiner grouped elements into triads of three with similar properties, where the middle element's atomic mass was roughly the average of the other two. John Newlands arranged elements in order of increasing atomic mass and noticed that properties repeated every eighth element — his Law of Octaves. This worked only for the lighter elements.
Mendeleev's breakthrough
Dmitri Mendeleev (1869) arranged the known elements by increasing atomic mass into rows and columns, grouping those with similar chemical properties. His genius was to leave gaps for undiscovered elements and to predict their properties. When gallium and germanium were later found, they matched his predictions closely, confirming his table.
Remember
- Döbereiner — triads of three similar elements.
- Newlands — Law of Octaves (repeat every 8th element).
- Mendeleev — arranged by atomic mass, left gaps, predicted new elements.
- Moseley — arranged by proton number (modern basis).
The modern table
Mendeleev sometimes had to swap elements out of mass order to fit the pattern. Henry Moseley solved this by showing that elements should be arranged in order of increasing proton (atomic) number, not atomic mass. This is the basis of the modern periodic table and removed the earlier anomalies.
Worked example
Why is tellurium (Te) placed before iodine (I) even though Te has a larger atomic mass? Because Moseley showed the correct order is by proton number: Te (52) comes before I (53). Their chemical properties then fall into the correct groups.