- The first classification of elements was s metals and non-metals. It served only limited purpose.

- After atomic masses (old term, atomic weight) of elements had been determined, it was thought to be their most fundamental property and attempts were made to correlate it to their other properties.

-Dobereiner grouped elements into triads. The atomic mass and properties of the middle element were mean of the other two. He could group only a few elements into triads. For example (i) Li, Na and K (ii) Ca, Sr and Ba (iii) Cl, Br and I.

- Newland tried to see the periodicity of properties and stated his law of octaves that, “When elements are arranged in the increasing order of their atomic weights every eighth element has properties similar to the first”. He could arrange elements up to calcium only out of more than sixty elements known then.

- Mendeleev observed correlation between atomic masses and other properties and stated his periodic law as, “The chemical and physical properties of elements are a periodic function of their atomic weights”.

- Mendeleev gave the first periodic table which is named after him which included all the known elements. It consists of seven horizontal rows called periods and numbered from 1 to 7. It has nine vertical columns called groups and numbered from zero to VIII.

- Main achievements of Mendeleev’s periodic table were (i) inclusion of all the known elements and (ii) prediction of new elements.

- Main defects of Mendeleev’s periodic table were (i) position of isotopes, (ii) anomalous pairs of elements like Ar and K and (iii) grouping of dissimilar elements and separation of similar elements.