Hydrogen is the lightest element. It is a colourless, odourless gas and about 14 times lighter than air. Hydrogen is a reactive element and little 'free' hydrogen gas is found on earth. Hydrogen is mostly combined with other elements to form compounds. Water is a compound of hydrogen and oxygen. The sun and stars are mostly made of hydrogen and this gas is also spread very thinly throughout space. For these reasons hydrogen is the most abundant element in the universe.
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