Question

multiple - choice question
initially, mendeleev organized elements by...
reactivity.
radioactivity.
atomic weight.
color.

Explanation:

Dmitri Mendeleev initially organized the elements in his periodic table by atomic weight. He noticed that when elements were arranged in order of increasing atomic weight, there were recurring patterns in their properties. Reactivity and radioactivity (which wasn't well - known in Mendeleev's time) are properties related to elements but not the basis of his initial organization. Color is a very superficial and not a fundamental property for organizing the entire set of elements in a scientific, systematic way like atomic weight.

Answer:

Atomic weight.