Substances containing atoms of only one type. e.g Na, Au, Mg etc. There are around 118 elements known to us. The first classification of elements was into Metals and Non-metals. Elements are classified to make the study easy.

Dobereiner’s Traids

When the elements were written in order of increasing atomic masses the atomic mass of the middle was the average of the atomic mass of the other two elements.

Elements Atomic Mass

  • Ca (40.1)
  • Sr (87.6)
  • Ba (136.3)

Limitations of Dobereiner’s Traids

Only three triads were recognised from the elements known at that time. Atomic mass of an element is the relative mass of its atom as compared with the mass of a Carbon-12 atom taken as 12 units

Newlands Law of Octave’s

  • Based on increasing atomic mass of elements.
  • Law of Octaves: When elements are arranged it was found that every eighth element had properties similar to that of the first. e.g properties of sodium and Lithium are the same.


  • Applicable only upto Calcium
  • Properties of new elements couldn’t fit in it.
  • In some cases, properties of the elements were not same as defined by the octave. eg: Fe was placed far away from Co and Ni.
  • Worked well only with lighter elements.

Mendeleev’s periodic table

Mendeleev’s Periodic Law :

  • The properties of elements are the periodic function of their atomic mass.
  • Mendeleev’s periodic table based on the chemical properties of elements. It contains vertical columns called groups and horizontal rows called periods.
  • For classifying he focussed on the oxides and hydrides formed by these elements. He arranged them according to their atomic mass. It was observed that not only the chemical but physical properties recurred at equal intervals.

Achievements of Mendeleev’s Periodic table

  • Elements with similar properties could be grouped together Some gaps place for isotopes.
  • No regular trend in atomic mass.
  • Co was placed before Ni.



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