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A simplified picture of a helium atom, with two protons (red), two neutrons (white), and two electrons (blue)

An atom is the basic unit of matter. All normal matter – everything that has mass – is made of atoms. This includes solids, liquids, and gases. The atom cannot be broken to parts by chemistry, so people once thought it was the smallest and simplest particle of matter. There are over 100 different kinds of atoms, called chemical elements. Each kind has the same basic structure, but a different number of parts.