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- Chemicals such as hydrogen (H), iron (Fe), sodium (Na), carbon (C), nitrogen (N), or oxygen (O), whose distinctly different atoms serve as the basic building blocks of all matter. There are 92 naturally occurring elements. Another 15 have been made in laboratories. Two or more elements combine to form compounds that make up most of the world's matter. See compound.
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- Atoms, Black Carbon, Chemical reaction, Compound, inorganic compound, Light-duty vehicles, molecule, Non-Methane Volatile Organic Compounds (NMVOCs), Organic compound, Oxidize, Volatile organic compounds (VOCs)