Mendeleeff arranged the elements in the increasing order of their atomic weights.
Salient Features :
1) Groups and sub-groups : There are 8 vertical columns called groups, represented by Roman numerals I to VIII. Each group is divided into two sub-groups A and B. The elements present in a group and its sub-group have similar properties.
2) Periods : The horizontal rows in Mendeleeffs table are called 'periods'. There are 7 periods, denoted by Arab numerals 1 to 7. A period comprises the entire range of elements after which properties repeat themselves.
3) Predicting the properties of missing elements : Based on the arrangement of the elements in the table, he predicted that some elements were missing and left blank spaces at the appropriate places in the table. He named those elements tentatively by adding the prefix 'eka' to the name of the element immediately above the each empty space. Eg : Eka-boron, Eka - aluminium, Eka - silicon were close to the observed properties of Scandium. Galium and Germanium respectively which were discovered later.
4) Correction of atomic weight: The correct placement of elements in Mendeleeffs periodic table helped in correcting the atomic masses of some elements like Beryllium, Indium, Gold.
5) Anomalous series : Some anomalous series of elements like 'Te' and I were observed in the table. The anomalous series contained elements with more atomic weight like (Te) placed before the element with less atomic weight like I.