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An atom is defined by the US Nuclear Regulatory Commission as "the basic building block of matter." Matter, or anything that occupies space, is made up of atoms. An atom is also the most basic unit of an element. Its name comes from the Greek word for indivisible, as atoms...

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An atom is defined by the US Nuclear Regulatory Commission as "the basic building block of matter." Matter, or anything that occupies space, is made up of atoms. An atom is also the most basic unit of an element. Its name comes from the Greek word for indivisible, as atoms were originally believed to be the smallest particles. Atoms usually consist of protons, neutrons, and electrons. Together, atoms can form molecules, which then become liquids, solids, or gases; many scientists think of atoms as "building blocks" of matter.

According to scientific research, atoms were created after the Big Bang approximately 13.7 billion years ago, and the theory of the atom dates as far back as 440 BC, when Greek philosopher and scientist Democritus theorized about the existence of atoms, likely expanding on the ideas of philosophers before him. In the centuries since, other scientists have pondered what atoms are and where they came from, and they eventually discovered the existence of protons, neutrons, and electrons.

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