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Our understanding of the atom has grown over a long time. As early as 400 BCE, a Greek philosopher named Democritus proposed that matter is composed of tiny, solid particles that are indivisible and indestructible. He called these particles

atomos,

or atoms. Democritus believed that atoms in different substances had different sizes and shapes, and these differences explained the variation in substances' properties.

After Democritus's early theorizing, it took more than two thousand years for a model of the atom based on experimental evidence to be developed. Between 1803 and 1807, English chemist and physicist John Dalton made experimental observations that led him to develop a new atomic theory. Dalton's model described atoms as solid, indivisible spheres.