History of atomic theory

In chemistry and physics, atomic theory is a theory of the nature of matter. It states that all matter is composed of atoms. The philosophical background of the atomic theory is called atomism. The theory applies to the common phases of matter, namely solids, liquids and gases, as directly experienced on Earth. Strictly speaking, it is not the appropriate theory for plasmas or neutron stars where unusual environments such as extremes of temperature or density prevent atoms from forming.

Philosophical atomism

Up until the beginning of the 19th century, atomic theory was mainly philosophical and not founded in scientific experimentation. The earliest known theories were developed in ancient India by Hindu, Buddhist and Jaina philosophers. The first philosopher who formulated ideas about the atom in a systematic manner was Kanada. Another Indian philosopher, Pakudha Katyayana, who also lived in the 6th century BC, had also propounded ideas about the atomic constitution of the material world. Indian atomists believed that an atom could be one of up to six elements, with each element having up to 24 properties. They developed detailed theories of how atoms could combine, react, vibrate, move, and perform other actions, and had particularly elaborate theories of how atoms combine, which explained how atoms first combine in pairs, and then group into trios of pairs, which are the smallest visible units of matter. They had also suggested the possibility of splitting an atom. (See Indian atomism for more details.)

Democritus and Leucippus, Greek philosophers in the 5th century BC, presented their own theory of atoms. The Greeks believed that atoms were all made of the same material but had different shapes and sizes, which determined the physical properties of the material. For instance, the atoms of a liquid were thought to be smooth, allowing them to slide over each other.

During the Middle Ages (the Islamic Golden Age), Islamic atomists developed atomic theories that represent a synthesis of both Greek and Indian atomism. Older Greek and Indian ideas were further developed by Islamic atomists, along with new Islamic ideas, such as the possibility of there being particles smaller than an atom. As Islamic influence began spreading through Europe, the ideas of Islamic atomism, along with the older ideas of Greek and Indian atomism, spread throughout Europe by the end of the Middle Ages.

Modern Atomic Theory

Birth of modern atomic theory

In 1803, the French chemist Joseph Louis Proust provided experimental proof of his law of definite proportions. This law basically states that if a compound is completely broken down into its constituent elements, then the masses of the constituents will always have the same proportions, no matter what the quantity of the original substance. John Dalton, a British chemist and Proust's contemporary, studied and expanded upon Proust's work to develop the law of multiple proportions: if two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small integers.

One set of combinations Dalton is believed to have studied involved nitrous oxide (NO) and oxygen (O₂). In one combination, these gases formed dinitrogen trioxide (N₂O₃), but when he repeated the combination with double the amount of oxygen (a ratio of 1:2 - small integers), they instead formed nitrogen dioxide (NO₂). The two reactions are illustrated below.

4NO + O₂ → 2N₂O₃

4NO + 2O₂ → 4NO₂

Dalton's efforts to explain this phenomenon led to him developing his theory of atoms. Dalton proposed that an element is composed of atoms of a single, unique type, and that although their shape and structure was immutable, atoms of different elements could combine to form more complex structures (chemical compounds).

In 1827, biologist Robert Brown observed that pollen grains floating in water constantly jiggled about for no apparent reason. In 1905, Albert Einstein theorised that this Brownian motion was caused by the water molecules continuously knocking the grains about, and developed a hypothetical mathematical model to describe it. This model was validated experimentally in 1911 by French physicist Jean Perrin, thus providing additional validation for atomic theory.

Discovery of subatomic particles

For much of this time, atoms were thought to be the smallest possible division of matter, until 1897 when J.J. Thomson discovered the electron through his work on cathode ray tubes. A cathode ray tube is a sealed glass cylinder in which two electrodes are separated by a vacuum. When a voltage is applied across the electrodes, cathode rays are generated, causing the tube to glow. Through experimentation, Thomson discovered that the negative charge could not be separated from the rays (by the application of magnetism), and that the rays could be deflected by an electrical field. He concluded that these rays, rather than being waves, were composed of negatively charged particles he called "corpuscles" (they would later be renamed electrons by other scientists), and that these corpuscles came from the very atoms of the electrode.

Thomson concluded that atoms were divisible, and that the corpuscles were their building blocks. To explain the overall neutral charge of the atom, Thomson proposed that the corpuscles were distributed in ring-like structures in a uniform sea or cloud of positive charge; this was the plum pudding model.

Since atoms were found to be actually divisible, physicists later invented a new term for indivisible particles: "elementary particles".

Discovery of the nucleus

Thomson's plum pudding model was disproved in 1909 by one of his students, Ernest Rutherford, who discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of their volume, which he assumed to be at their very center.

In the gold foil experiment, alpha particles (emitted by polonium) were shot through a sheet of gold (striking a fluorescent screen on the other side). Given the very small mass of the electrons, the high mass and momentum of the alpha particles and the uniform distribution of positive charge of the plum pudding model, the experimenters expected all the alpha particles to either pass through without significant deflection or be absorbed. To Rutherford's astonishment, about 1 in 8000 of the alpha particles experienced heavy deflection (by more than 90 degrees). This led Rutherford to propose the planetary model of the atom in which pointlike electrons orbited in the space around a massive compact nucleus like planets orbiting the Sun.

Ten years later, Rutherford discovered that he could transmute one element into another by bombarding it with alpha particles. In each of these cases, hydrogen nuclei were emitted. By comparing nuclear masses to charges he found that the positive charge of any atom could be equated to that of an integer number of hydrogen nuclei. Rutherford had discovered the proton (a hydrogen nucleus is basically a proton). Further experimentation by Rutherford found that the nuclear mass of most atoms exceeded that of the protons it possessed; this led him to postulate first the existence of "nuclear electrons" and finally neutrons, whose existence would be proven in 1932 by James Chadwick.

Quantum physical models of the atom

The planetary model of the atom still had shortcomings. Firstly, a moving electric charge emits electromagnetic waves; according to the Larmor formula in classical electromagnetism, an orbiting charge would steadily lose energy and spiral towards the nucleus, colliding with it in a tiny fraction of a second. Another phenomenon the model did not explain was why excited atoms only emit light with certain discrete spectra.

J.J. Thomson had been fully aware that a moving electric charge would emit electromagnetic waves, hence why he did not originally propose the single-planetary model and instead had proposed the plum pudding model with rings of electrons (balanced rings of electrons do not change dipole moment as they rotate, which partially solved the radiation problem; however individual electrons were still accelerated, so the plum pudding model was not perfect). Thomson later formed a planetary model based on Boscovich's theory, which was later taken up and elaborated upon by Niels Bohr.

Quantum theory revolutionized physics at the beginning of the 20th century when Max Planck and Albert Einstein postulated that light energy is emitted or absorbed in fixed amounts known as quanta. In 1913, Niels Bohr incorporated this idea into Bohr model of the atom, in which the electrons could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy. They were not allowed to spiral into the nucleus, because they could not lose energy in a continuous manner; they could only make quantum leaps between fixed energy levels. Bohr's model was extended by Arnold Sommerfeld in 1916 to include elliptical orbits, using a quantization of generalized momentum.

The ad hoc Bohr-Sommerfeld model was extremely difficult to use, but it made impressive predictions in agreement with certain spectral properties. However, the model was unable to explain multielectron atoms, predict transition rates or describe fine and hyperfine structure.

As research into quantum physics progressed, a new model of the atom was proposed wherein the nucleus was surrounded by a cloud of electrons whose precise positions could only be guessed with the aid of probabilistic equations (see electron configuration).

References

External links

Ancient Atomism