There are eight vertical columns in Mendeleeff’s periodic table called as groups. They are represented by Roman numerals I to VIII. Elements present in a given vertical column (group) have similar properties. Each group is divided into two sub-groups ‘A’ and ‘B’. The elements within any sub-group resemble one another to a great extent. For example, sub-group IA elements called ‘alkali metals’ (Li, Na, K, Rb, Cs) resemble each other very much in their properties.

The horizontal rows in Mendeleeff’s periodic table are called periods. There are seven periods in the table, which are denoted by Arabic numerals 1 to 7. Elements in a period differ in their properties from one another. A period comprises the entire range of elements after which properties repeat themselves

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.

Mendeleeff believed that some new elements would be discovered definitely. He predicted the properties of these new additional elements in advance purely depending on his table. His predicted properties were almost the same as the observed properties of those elements after their discovery.

He named those elements tentatively by adding the prefix‘eka’ (eka is a Sanskrit word for numeral one) to the name of the element immediately above each empty space. The predicted properties of elements namely eka-boron, eka-aluminium and eka-silicon were almost same as those of scandium, gallium and germanium respectively which were discovered later.