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Atoms of H, Li, ____, and K each have one valence electron.
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hydrogen (H)
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Li, Na, and ____ are highly reactive alkali metals.
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iron (Fe)
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When an electric current is passed through ____ it glows red.
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neon (Ne)
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A dense, radioactive, noble gas is ____.
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oxygen (O)
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The lightest and most abundant element in the Universe is ____.
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potassium (K)
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About 21% of the Earth's atmosphere is ____.
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radon (Rn)
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The metal component of the hemoglobin protein is ____.
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sodium (Na)
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An isotope with a half-life of 4.5 billion years is ____-238.
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uranium (U)
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