The first classification of elements was as metals and non-metals. After the discovery of atomic mass, it was thought to be the fundamental property of elements and attempts were made to correlate it to their other properties.
John 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.
Newlands tried to see the periodicity of properties and stated his law of octaves as 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 then known.
Mendeleev observed the correlation between atomic weight 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 them from 1 to 7. It has eight vertical columns called groups and numbered them from I to VIII.
Moseley discovered that atomic number and not atomic mass is the fundamental property of elements. In the light of this the periodic law was modified to - The chemical and physical properties of elements are periodic functions of their atomic numbers. This is the Modern Periodic Law.
Modern Periodic Table is based upon atomic number. Its long form has been accepted by IUPAC. It has seven periods (1 to 7) and 18 groups (1 to 18). Elements belonging to same group have same number of valence electrons and thus show same valency and similar chemical properties.
Arrangement of elements in the periodic table shows periodicity. Atomic radii and metallic character increase in a group from top to bottom and in a period decrease from left to right.