rate at which the elements chemically react. hardness of the elements. structure of the atoms of the elements. radioactivity of the atoms of the elements.
I gave my students this multiple choice question: The periodic table of the elements is systematically organized according to the

Only one answer is correct among these 4 choices:
- rate at which the elements chemically react.
- hardness of the elements.
- structure of the atoms of the elements.
- radioactivity of the atoms of the elements.

Could you tell me which one is correct?

structure of the atoms of the elements.