66.

Give a brief discussion of the Mendeleev's Periodic table. 

Mendeleev arranged all the known elements (63 at that time) in horizontal rows and vertical columns in order of increasing atomic masses in the form of table known as Periodic table. Mendeleev’s table has few vertical columns called groups and a few horizontal rows called periods. Mendeleev left certain gaps in his table and predicted that new elements would be discovered to fill up the gaps.
The Mendeleev's modified periodic table consists of:

(i) Nine vertical columns called groups. These are numbered from I to VIII and zero (The members of the zero group were not discovered at the time of Mendeleev). Each group from I to VII, is further sub-divided into two sub-groups-A and B. Group VIII consists of three sets, each containing three elements. Group zero consists of noble gases.

(ii) Seven horizontal rows are called periods. These are numbered from I to VII. First period contains two elements, second and third periods contain eight elements each. These periods are called short periods. Fourth and fifth periods contain eighteen elements each. These periods are called long periods. Sixth period contains thirty two elements and is called longest period. Seventh period is incomplete and contains nineteen elements (At. No. 87-105).