Final answer:
Rehanium is not an elementary substance because it does not exist as an element on the periodic table.
Step-by-step explanation:
Rehanium is not an elementary substance. An elementary substance, also known as a chemical element, is a type of pure substance that cannot be broken down into simpler substances by chemical means. Examples of elementary substances include hydrogen, oxygen, carbon, and gold. Rehanium does not exist as an element on the periodic table and therefore cannot be considered an elementary substance.
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