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Democritus taught the theory of atomism, which held the belief that indivisible and indestructible atoms are the basic components of all matter in the universe in about 400 B.C.E. He lived in Greece.

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All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties. Atoms cannot be subdivided, created, or destroyed. This theory was made in 1808 and was in the United Kingdom.

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Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. He was from Manchester and delivered his address in London in 1897.

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Rutherford's contribution was the discovery that atoms contain a positively charged nucleus much smaller than the actual atom and also postulated the nuclear structure of the atom, discovered alpha and beta rays, and proposed the laws of radioactive decay.He made his discovery in 1911 in Manchester.

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In 1913 Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values. Electrons move around a nucleus, but only in prescribed orbits, and If electrons jump to a lower-energy orbit, the difference is sent out as radiation. He was from Copenhagen, Denmark.

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Erwin Schrödinger theorized that the behavior of electrons within atoms could be explained by treating them mathematically as matter waves (quantum mechanical model). He made this discovery in 1926 in Vienna.