Mendeleev saw trends in the physical and chemical properties of elements when he organized them by atomic number physical properties atomic weight density

How has the scientific community addressed the safety of chemicals? a. chemicals are repeatedly tested, even those that have existed for a long time. b. existing chemicals are tested if they have never been tested before. c. chemicals are tested if they are suspected to have caused a problem. d. only new chemicals are tested.

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How are isotopes of the same chemical element alike? how are they different?