Development of the periodic table

• Before the discovery of protons, neutrons and electrons, scientists tried to arrange elements in order of their atomic weights (now known as relative atomic masses).

• They noticed that elements with similar properties appeared at regular intervals, but early versions of the periodic table were incomplete.

• Because only a few elements were known, many were placed in inappropriate groups when arranged strictly by atomic weight

Newlands' Octaves

• In 1869, Russian chemist Dmitri Mendeleev created the first useful version of the periodic table.

• He arranged elements mainly by increasing atomic weight, but unlike earlier scientists, he also considered chemical properties.

• When properties did not fit the pattern, Mendeleev changed the order of some elements so that those with similar properties were in the same vertical groups.

• He left gaps in his table for undiscovered elements, predicting their atomic weights and properties.

• When these missing elements were later discovered (e.g. gallium and germanium), their properties closely matched Mendeleev’s predictions.

• This provided strong evidence that his arrangement was correct and meaningful, and his work became the foundation for the modern periodic table.
  

Mendeleev's 1871 table

Later Developments and Modern Understanding

• With the discovery of subatomic particles, scientists gained a clearer understanding of atomic structure.

• The elements were later rearranged in order of atomic (proton) number, not atomic weight.

• The discovery of isotopes explained why ordering purely by atomic weight sometimes caused elements to be in the wrong place.

For example, argon has a higher atomic mass than potassium, but it appears before potassium in the modern table because its atomic number is smaller (18 vs 19).