Atom

History

Historical theories

Democritus and Leucippus, Greek philosophers in the 5th century BC, presented the first theory of atoms (see article atomism for more details). They held that each atom had a different shape, like a pebble, that governed the atom's properties. Dalton and Avogadro rediscovered the works of Democritus and Leucippus and suggested in the 19th century that matter was made up of atoms, but they knew nothing of their structure. This theory was conflicting with the theory of infinite divisibility, which states that matter can always be divided into smaller parts.

Related Topics:
Democritus - Leucippus - Greek philosophers - 5th century BC - Atomism - Dalton - Avogadro - 19th century - Infinite divisibility - Matter

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The controversy settled down in 1911 by Perrin when he discovered the meta-particle we nowadays call an atom. Jean Perrin thought that he had found the "atomos" that Democritus talked about and so named his particles atoms.

Related Topics:
1911 - Perrin - Democritus

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Through this time, atoms were thought to be the smallest possible piece of matter. However, it was later shown that atoms are made up of subatomic particles. Thomson's experiments discovered the electron, the first of the subatomic particles to be discovered. This showed that atoms are actually divisible, and not the indivisible "atomos" Democritus talked about. Physicists later invented a new term for indivisible units, namely elementary particles since the word atom had already been taken and come into common use.

Related Topics:
Matter - Subatomic particle - Thomson - Electron - Democritus - Elementary particles

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At first, it was believed that the electrons were distributed more or less uniformly in a sea of positive charge (the plum pudding model). However, an experiment conducted a few years later by Rutherford demonstrated that atoms are mostly empty space, with a lot of mass concentrated in a nucleus. In the gold foil experiment, he shot alpha particles (emitted by polonium) through a a sheet of gold. He observed that most of the particles passed straight through the sheet without deflection (striking a fluorescent screen on the other side), but that, surprisingly, a small number were bounced right back (having come close to a nucleus). This led to the planetary model of the atom, in which the electrons orbited the nucleus like the planets orbiting the sun.

Related Topics:
Plum pudding model - Gold foil experiment - Alpha particles - Polonium

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The nucleus was later discovered to contain protons, and further experimentation by Rutherford found that the nuclear mass of most atoms surpassed the number of protons it possessed; this led him to postulate the existence of neutrons, whose existence would be proved in 1932 by James Chadwick.

Related Topics:
Proton - Neutrons - James Chadwick

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Later, experiments by Max Planck and Albert Einstein demonstrated that energy is transferred in tiny fixed amounts known as quanta. This led Bohr to propose an updated model, wherein the electrons orbited the nucleus in fixed circles. Because their energy could only change by fixed quantities, they couldn't move closer to or farther from the nucleus in spirals; they could only make quantum leaps from one circle to the next.

Related Topics:
Max Planck - Albert Einstein - Quanta - Quantum leaps

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Study of atoms

The study of atoms was done by largely indirect means through the 19th century and early 20th century. In recent years, however, new techniques have made the identification and study of atoms easier and more accurate. The electron microscope, invented in 1931, has allowed pictures to be taken of actual, individual atoms. Atomic force microscopy is another technique by which individual atoms can be visualized. Methods also exist to identify atoms and compounds. Elemental analysis allows the exact identification of the types and amounts of atoms in a substance.

Related Topics:
19th century - 20th century - Electron microscope - 1931 - Atomic force microscopy - Elemental analysis

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