Explanation:
Mendeleev placed Cobalt (atomic mass 58.9) before Nickel (atomic mass 58.7) to maintain chemical similarities, violating his own periodic law based on atomic mass. Isotopes weren't an issue then, and H's position was ambiguous, but the Co/Ni placement is a classic anomaly of his atomic mass-based table.
Explanation:
PVC (polyvinyl chloride) is a polymer of vinyl chloride (CH₂=CHCl). It is used in pipes, electrical insulation, and flooring. It can be rigid (uPVC) or flexible with plasticizers. It is a thermoplastic. Chlorate/carbon/carbide are not correct expansions.
Explanation:
Uranium (U) is a naturally occurring heavy metal that is radioactive. Its most common isotopes are U-238 and U-235. Aluminum, iron, and calcium are stable, non-radioactive elements under normal conditions, though they may have synthetic radioactive isotopes.
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