<h2>Answer:</h2>
Mendeleev grouped elements with similar chemical properties (especially valency) together and noted their mass as well. At first he only wrote down 9 elements:
Cl 35.5, K 39, Ca 40
Br 80, Rb 85, Sr 88
I 127, Cs 133, Ba 137
He added more elements to his first period table little by little, putting them into groups by valency and sorting them within their group by mass. He noted that the mass in most cases also increases by group within a period.
On 6 March 1869, he presented his work "The Dependence between the Properties of the Atomic Weights of the Elements" for the first time. It stated that:
The elements, if arranged according to their atomic weight, exhibit an apparent periodicity of properties.
Elements which are similar regarding their chemical properties either have similar atomic weights (e.g., Pt, Ir, Os) or have their atomic weights increasing regularly (e.g., K, Rb, Cs).
The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F.
The elements which are the most widely diffused have small atomic weights.
The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
