Answer:

Mendeleev arranged the elements in his periodic table by atomic weight. Mendeleev classified the elements into groups and periods based on atomic weight. Mendeleev grouped elements with similar properties.

Nonetheless, he didn’t keep a promise he made. He noticed that if the elements were ordered by increasing atomic weight, some elements did not fit.

In this way, he occasionally forgot the atomic weight order. For example, iodine has a smaller atomic mass than tellurium.

He chose tellurium over iodine in Group 7 because its properties are so similar to those of fluorine, chlorine, and bromine.