RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY
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Helium and hydrogen molecules. Computer artwork of molecules of helium and hydrogen, the two lightest and most abundant elements in the universe. Helium (He) is a monatomic chemical element, meaning every molecule of the gas comprises only one atom. Each atom consists of a nucleus (orange), surrounded by an electron cloud (pink), which contains two electrons orbiting in a single energy level. Hydrogen (H2), however, is a diatomic molecule. Each atom has a nucleus (dark green) surrounded by an electron cloud (light green), containing only one electron. Therefore in a gaseous state (as is seen here), two atoms will combine to form a covalently bonded molecule, which has two electrons orbiting the nucleus, increasing its stability.
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