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Elements

    Just as hydrogen, oxygen, and carbon can from the compounds carbon dioxide and water, atoms of the same type can also combine with one another to form molecules. For example, hydrogen atoms can pair up, as can oxygen atoms. For reasons we will consider later, carbon atoms form much large groups, leading to substances such as diamond, graphite, and buckminsterfullerene.

    Because pure hydrogen and oxygen each contains only one type of atom, the substances are called elemental substances or, more commonly, elements. Elements are substances that contain only one kind of atom; a sample of iron contains many atoms, but they are all iron atoms. Samples of certain pure elements do contain molecules; for example, hydrogen gas contains H-H (usually wrrittrn H2) molecules, and oxygen gas contains O-O (O2) molecules. However, any pure sample of an element contains only atoms of that element, never any atoms of any other element.

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