History of Atomic Theory

Learn how atomic theory has changed over time.
Learn how atomic theory has changed over time. artpartner-images / Getty Images
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A Brief History of Atomic Theory

Atomic theory describes the nature of atoms, the building blocks of matter.
Atomic theory describes the nature of atoms, the building blocks of matter. artpartner-images / Getty Images

Atomic theory is a scientific description of the nature of atoms and matter. It combines elements of physics, chemistry, and mathematics. According to the modern theory, matter is made of tiny particles called atoms, which are in turn made up of subatomic particles. Atoms of a given element are identical in many respects and different from atoms of other elements. Atoms combine in fixed proportions with other atoms to form molecules and compounds.

The theory has evolved over time, from the philosophy of atomism to modern quantum mechanics. Here is a brief history of atomic theory.

The Atom and Atomism

The 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 consisted of discrete particles. However, the theory was one of many explanation for matter and was not based on empirical data. In the fifth century B.C., Democritus proposed matter consisted of indestructible, indivisible units called atoms. The Roman poet Lucretius recorded the idea, so it survived through the Dark Ages for later consideration.

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Dalton's Atomic Theory

Up to the 18th century, there was no experimental evidence for the existence of atoms. No one knew how finely matter could be divided.
Up to the 18th century, there was no experimental evidence for the existence of atoms. No one knew how finely matter could be divided. Aeriform / Getty Images

It took until the end of the 18th century for science to provide concrete evidence of the existence of atoms. Antoine Lavoisier formulated the law of conservation of mass in 1789, which states the mass of the products of a reaction is the same as the mass of reactants. Joseph Louis Proust proposed the law of definite proportions in 1799, which states the masses of elements in a compound always occur in the same proportion. These theories did not reference atoms, yet John Dalton built upon them to develop the law of multiple proportions, which states the ratio of masses of elements in a compound are small whole numbers. Dalton's law of multiple proportions drew from experimental data. He proposed each chemical element consists of a single type of atoms 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 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 element and made clear there was a distinction between atoms and molecules.

Another significant contribution to atomic theory was made in 1827 by botanist Robert Brown, who noticed dust particles floating in water seemed to move randomly for no known reason. In 1905, Albert Einstein postulated the 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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Plum Pudding Model and Rutherford Model

Rutherford proposed a planetary model of atoms, with electrons orbiting a nucleus like planets orbiting a star.
Rutherford proposed a planetary model of atoms, with electrons orbiting a nucleus like planets orbiting a star. MEHAU KULYK/SCIENCE PHOTO LIBRARY / Getty Images

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 the positive charge of an atom and most of its mass was at the center or nucleus of an atom. He described a planetary model in which electrons orbited a small positive-charged nucleus.

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Bohr Model of the Atom

According to the Bohr model, electrons orbit the nucleus at discrete energy levels.
According to the Bohr model, electrons orbit the nucleus at discrete energy levels. MARK GARLICK/SPL / Getty Images

Rutherford was on the right track, but his model could not 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 electron 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.

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Quantum Atomic Theory

According to modern atomic theory, an electron could be anywhere in an atom, but it's most probable it is in an energy level.
According to modern atomic theory, an electron could be anywhere in an atom, but it's most probable it is in an energy level. Jamie Farrant / Getty Images

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 wave-like behavior of moving particles, which Erwin Schrodinger described using Schrodinger's equation (1926). This, in turn, led to the Heisenberg uncertainty principle (1927), which states 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 greatest probability in an atomic orbital or energy level. Rather then the circular orbits of Rutherford's model, modern atomic theory describes orbitals that may be spherical, dumb bell shaped, etc. For atoms with a high number of electrons, relativistic effects come into play, since the particles are moving a speeds that are a fraction of the speed of light. Modern scientists have found smaller particles that make up the protons, neutrons, electrons, although the atom remains the smallest unit of matter that cannot be divided using any chemical means.