Q. Which is the lightest and most abundant chemical element, constituting roughly 75 % of the Universe’s elemental mass?
Answer: Hydrogen
Notes: The lightest and most abundant chemical element in the Universe is hydrogen. It constitutes about 75% of the elemental mass in the Universe. Hydrogen is also the first element in the periodic table, with the atomic number 1. It is a highly reactive element, and it can form a wide variety of compounds with other elements.

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