A Brief History of Atomic Theory

The Atom and Atomism

Atomic theory originated as a philosophical concept in ancient India and Greece. The word "atom" comes from the ancient Greek word atomos, which means indivisible. According to atomism, matter consists of discrete particles. However, the theory was one of many explanations for matter and wasn't based on empirical data. In the fifth century BCE, Democritus proposed that matter consists of indestructible, indivisible units called atoms. The Roman poet Lucretius recorded the idea, so it survived through the Dark Ages for later consideration.

Dalton's Atomic Theory

It took until the end of the 18th century for science to provide concrete evidence of the existence of atoms. In 1789, Antoine Lavoisier formulated the law of conservation of mass, which states that the mass of the products of a reaction is the same as the mass of the reactants. Ten years later, Joseph Louis Proust proposed the law of definite proportions, which states that the masses of elements in a compound always occur in the same proportion.

These theories didn't reference atoms, yet John Dalton built upon them to develop the law of multiple proportions, which states that the ratios of masses of elements in a compound are small whole numbers. Dalton's law of multiple proportions drew from experimental data. He proposed that each chemical element consists of a single type of atom that could not be destroyed by any chemical means. His oral presentation (1803) and publication (1805) marked the beginning of the scientific atomic theory.

In 1811, Amedeo Avogadro corrected a problem with Dalton's theory when he proposed that equal volumes of gases at equal temperature and pressure contain the same number of particles. Avogadro's law made it possible to accurately estimate the atomic masses of elements and made a clear distinction between atoms and molecules.

Another significant contribution to atomic theory was made in 1827 by botanist Robert Brown, who noticed that dust particles floating in water seemed to move randomly for no known reason. In 1905, Albert Einstein postulated that Brownian motion was due to the movement of water molecules. The model and its validation in 1908 by Jean Perrin supported atomic theory and particle theory.

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Basic Model of the Atom and Atomic Theory

By Anne Marie Helmenstine, Ph.D.

Plum Pudding Model and Rutherford Model

Up to this point, atoms were believed to be the smallest units of matter. In 1897, J.J. Thomson discovered the electron. He believed atoms could be divided. Because the electron carried a negative charge, he proposed a plum pudding model of the atom, in which electrons were embedded in a mass of positive charge to yield an electrically neutral atom.

Ernest Rutherford, one of Thomson's students, disproved the plum pudding model in 1909. Rutherford found that the positive charge of an atom and most of its mass were at the center, or nucleus, of an atom. He described a planetary model in which electrons orbited a small, positive-charged nucleus.

Bohr Model of the Atom

Rutherford was on the right track, but his model couldn't explain the emission and absorption spectra of atoms, nor why the electrons didn't crash into the nucleus. In 1913, Niels Bohr proposed the Bohr model, which states that electrons only orbit the nucleus at specific distances from the nucleus. According to his model, electrons couldn't spiral into the nucleus but could make quantum leaps between energy levels.

Quantum Atomic Theory

Bohr's model explained the spectral lines of hydrogen but didn't extend to the behavior of atoms with multiple electrons. Several discoveries expanded the understanding of atoms. In 1913, Frederick Soddy described isotopes, which were forms of an atom of one element that contained different numbers of neutrons. Neutrons were discovered in 1932.

Louis de Broglie proposed a wavelike behavior of moving particles, which Erwin Schrödinger described using Schrödinger's equation (1926). This, in turn, led to Werner Heisenberg's uncertainty principle (1927), which states that it's not possible to simultaneously know both the position and momentum of an electron.

Quantum mechanics led to an atomic theory in which atoms consist of smaller particles. The electron can potentially be found anywhere in the atom but is found with the greatest probability in an atomic orbital or energy level. Rather than the circular orbits of Rutherford's model, modern atomic theory describes orbitals that may be spherical, dumbbell-shaped, etc. For atoms with a high number of electrons, relativistic effects come into play, since the particles are moving at a fraction of the speed of light.

Modern scientists have found smaller particles that make up the protons, neutrons, and electrons, although the atom remains the smallest unit of matter that can't be divided using chemical means.