Question

after mendeleev organized the periodic table by atomic weight, he noticed a pattern of....?
reactivity.
color.
smell.
radioactivity.

Explanation:

Mendeleev's periodic table was based on atomic weight. He noticed that elements with similar chemical properties (which are related to reactivity) repeated at regular intervals. Color, smell, and radioactivity (radioactivity was not well - known in Mendeleev's time) were not the main patterns he observed.

Answer:

reactivity.