What did Mendeleev conclude when constructing the periodic table?

Answer 1

Mendeleev noticed that a pattern of properties appeared when he arranged the elements in the order of increasing atomic mass.He found that the properties repeated regulary,

Answer 2

Dmitri Mendeleev concluded that when elements are arranged in order of increasing Atomic Mass, their properties repeat periodically. He left gaps for undiscovered elements and predicted the properties of these missing elements based on their neighboring elements. This led to the development of the modern Periodic Table.