Atoms come in a variety of types, called elements, each with a unique set of physical and chemical properties. An element is defined by the number of protons in its atoms, known as its atomic number. The periodic table organizes the elements by atomic number and provides other key information about them such as their names and chemical symbols.

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“Galaxy Cluster Abell 2218” by NASA, licensed under a Creative Commons Attribution License (CC BY 2.0):
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“Liquid oxygen in a beaker (cropped and retouched)” by Nika Glover [Public domain], accessed via Wikimedia Commons:
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“Oxygen Tanks” by Todd Van Hoosear, licensed under a Creative Commons Attribution License (CC BY-SA 2.0):
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“Gold nugget (placer gold) 1 (17001285916)” by James St. John, licensed under a Creative Commons Attribution License (CC BY 2.0):
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“Gold necklace with coin pendants MET DP107044” by the Metropolitan Museum of Art [Public domain], accessed via Wikimedia Commons:
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“Xenon-glow” by Jurii, licensed under a Creative Commons Attribution License (CC BY 3.0):
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“Congrats bqt” by Pioneer Balloon Company [Public domain], accessed via Wikimedia Commons:
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“Iron electrolytic and 1cm3 cube” by Alchemist-hp, licensed under a Creative Commons Attribution License (CC BY-NC-ND 3.0):
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