(i) Different elements discovered in different times were found to possess different physical and chemical properties. But, it was difficult to study all the information or properties of these elements. Scientists started discovering some pattern in their properties to classify all the known elements in order to make their study easier. So, elements are classified on the basis of their similarities and dissimilarities in properties.
(ii) Noble gases like He, Ne, Ar, etc. were not discovered at the time of Mendeleev'. They were discovered later as they are inert and present in extremely low concentrations in our atmosphere. So, after their discovery, they were placed in a new group (18) in Modern PeriodicTable.
