In the beginning, the Atomic theory was explained as matter is composed of tiny, indivisible particles in constant motion which was proposed in the 5th cent. B.C. by the Greek philosophers Leucippus and Democritus and was adopted by the Roman Lucretius. However, Aristotle did not accept the theory, and it was ignored for many centuries. Interest in the atomic theory was revived during the 18th cent. following work on the nature and behaviour of gases. The later atomic models by John Dalton, J.J. Thomson, Earnest Rutherford, Neil Bohr and by many other scientists helped in further reviving the atomic theory and also abolished the myth that the atom is the Fundamental unit of matter and is indivisible.
Modern atomic theory begins with the work of John Dalton, published in 1808. He held that all the atoms of an element are of exactly the same size and weight and are in these two respects unlike the atoms of any other element. He stated that atoms of the elements unite chemically in simple numerical ratios to form compounds. The best evidence for his theory was the experimentally verified law of multiple proportions which gives a relation between the weights of two elements that combine to form different compounds. Evidence for Dalton’s theory also came from Michael Faraday’s law of electrolysis. A major development was the periodic table, devised simultaneously by Dmitri Mendeleev and Ajan Reginald, which arranged atoms of different elements in order of increasing atomic weight so that elements with similar chemical properties fell into groups. By the end of the 19th cent. it was generally accepted that matter is composed of atoms that combine to form molecules. In the beginning of the modern atomic theory, John Dalton came up with his Atomic theory with points like all matter is made of atoms that are indestructible and indivisible, that all atoms of a given element are identical in mass and properties, that compounds are formed by a combination of two or more different kinds of atoms, that a chemical reaction is a rearrangement of atoms. Dalton’s model was accepted for many decades to come but was later discarded with the braking of the atom and the discovery of the first subatomic particle called as the electron.
In 1897, Thomson showed that cathode rays were composed of a previously unknown negatively charged particle, and thus he is credited with the discovery and identification of the electron and in a broader sense, with the discovery of the first subatomic particle. Thomson is also credited with finding the first evidence for isotopes of a stable (non-radioactive) element in 1913, as part of his exploration into the composition of canal rays (positive ions). He invented the mass spectrometer. To discover the negatively charged particle electron, Thomson, In May–June 1897, investigated whether or not the rays could be deflected by an electric field. Previous experimenters had failed to observe this, but Thomson believed their experiments were flawed because their tubes contained too much gas. Thomson constructed a cylindrical glass tube with a near-perfect vacuum. At the start of the tube was the cathode from which the rays projected. The rays were sharpened to a beam by two metal slits – the first of these slits doubled as the anode, the second was connected to the earth. The beam then passed between two parallel aluminium plates, which produced an electric field between them when they were connected to a battery. The end of the tube was a large sphere where the beam would impact on the glass, created a glowing patch. Thomson pasted a scale to the surface of this sphere to measure the deflection of the beam. Note that any electron beam would collide with some residual gas atoms within the Cathode ray tube, thereby ionizing them and producing electrons and ions in the tube(Space charge). In previous experiments this space charge electrically screened the externally applied electric field. However, in Thomson’s Cathode ray tube the density of residual atoms was so low that the space charge from the electrons and ions was insufficient to electrically screen the externally applied electric field, which permitted Thomson to successfully observe electrical deflection. When the upper plate was connected to the negative pole of the battery and the lower plate to the positive pole, the glowing patch moved downwards, and when the polarity was reversed, the patch moved upwards. With this experiment, Sir J.J. Thomson discovered the negatively charged electron. Later in 1906, Thomson originally believed that the hydrogen atom must be made up of more than two thousand electrons, to account for its mass. An atom made of thousands of electrons would have a very high, negative electric charge. This was not observed, as atoms are usually uncharged. In 1906 Thomson suggested that atoms contained far fewer electrons, a number roughly equal to the atomic number. This is only one electron in the case of hydrogen, far fewer than the thousands originally suggested. These electrons must have been balanced by some sort of positive charge. The distribution of charge and mass in the atom was unknown. Thomson proposed a ‘plum pudding’ model, with positive and negative charge filling a sphere only one ten billionth of a metre across. This plum pudding model was generally accepted. Even Thomson’s student Rutherford, who would later prove the model incorrect, believed in it at the time.
By 1911 the components of the atom had been discovered. The atom consisted of subatomic particles called protons and electrons. However, it was not clear how these protons and electrons were arranged within the atom. J.J. Thomson suggested the”plum pudding” model. In this model the electrons and protons are uniformly mixed throughout the atom but in the same year, Sir Ernest Rutherford overthrew the so called “Plum Pudding” model by J.J. Thomson and proposed his own theory with the help of an experiment known as the Alpha scattering model. This model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun. The Rutherford atomic model has been alternatively called the nuclear atom, or the planetary model of the atom. The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin gold foil also called the gold foil experiment in 1911. The diagram shows a simplified plan of his gold foil experiment. A radioactive source capable of emitting alpha particles (i.e., positively charged particles more than 7,000 times as massive as electrons) was enclosed within a protective lead shield. The radiation was focused into a narrow beam after passing through a slit in a lead screen. A thin section of gold foil was placed in front of the slit, and a screen coated with zinc sulphide to render it fluorescent served as a counter to detect alpha particles. As each alpha particle struck the fluorescent screen, it would produce a burst of light called a scintillation, which was visible through a viewing microscope attached to the back of the screen. The screen itself was movable, allowing Rutherford and his associates to determine whether or not any alpha particles were being deflected by the gold foil. Most alpha particles were observed to pass straight through the gold foil, which implied that atoms are composed of large amounts of open space. Some alpha particles were deflected slightly, suggesting interactions with other positively charged particles within the atom. Still other alpha particles were scattered at large angles, while a very few even bounced back toward the source. Only a positively charged and relatively heavy target particle, such as the proposed nucleus, could account for such strong repulsion. The negative electrons that balanced electrically the positive nuclear charge were regarded as travelling in circular orbits about the nucleus. The electrostatic force of attraction between electrons and nucleus was likened to the gravitational force of attraction between the revolving planets and the Sun. Most of this planetary atom was open space and offered no resistance to the passage of the alpha particles. But there were a few drawbacks in the Rutherford’s atomic theory like as an electron revolves around the nucleus, it gets accelerated towards the nucleus. According to the electromagnetic theory, an accelerating charged particle must emit radiation, and lose energy. Because of this loss of energy, the electron would slow down, and will not be able to withstand the attraction of the nucleus. As a result, the electron should follow a spiral path, and ultimately fall into nucleus. If it happens then the atom should collapse in about 10-8 second. But, this does not happen: atoms are stable indicating that there is something wrong with this model. Also, Rutherford’s model doesn’t say anything about the arrangement of electrons inside the atom.
Two years later, In 1913 Another renowned scientist Sir Neil Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. He also took help from the Plank’s constant ’h’ (6.62606957×10-34 m2 kg/s) that was found in 1900 by the famous scientist Max Plank while explaining the Black Body Radiation(Explained below). The motion of the electrons in the Rutherford model was unstable because of the reason explained above. So in order to solve the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump. Bohr’s starting point was to realize that classical mechanics by itself could never explain the atom’s stability. A stable atom has a certain size so that any equation describing it must contain some fundamental constant or combination of constants with a dimension of length. The classical fundamental constants–namely, the charges and the masses of the electron and the nucleus–cannot be combined to make a length. Bohr noticed, however, that the quantum constant formulated by the German physicist Max Planck has dimensions which, when combined with the mass and charge of the electron, produce a measure of length. Numerically, the measure is close to the known size of atoms. This encouraged Bohr to use Planck’s constant in searching for an all new Atomic theory. As explained above Planck had introduced his constant in 1900 in a formula explaining the light radiation emitted from heated bodies. According to classical theory, comparable amounts of light energy should be produced at all frequencies. This is not only contrary to observation but also implies the absurd result that the total energy radiated by a heated body should be infinite. Planck postulated that energy can only be emitted or absorbed in discrete amounts, which he called quanta (the Latin word for “how much”). The energy quantum is related to the frequency of the light by a new fundamental constant, h. When a body is heated, its radiant energy in a particular frequency range is, according to classical theory, proportional to the temperature of the body. With Planck’s hypothesis, however, the radiation can occur only in quantum amounts of energy. If the radiant energy is less than the quantum of energy, the amount of light in that frequency range will be reduced. Planck’s formula correctly describes radiation from heated bodies. Planck’s constant has the dimensions of action, which may be expressed as units of energy multiplied by time, units of momentum multiplied by length, or units of angular momentum. For example, Planck’s constant can be written as h=6.6×10-34 joule seconds. Using Planck’s constant, Bohr obtained an accurate formula for the energy levels of the hydrogen atom. He postulated that the angular momentum of the electron is quantized i.e., it can have only discrete values. He assumed that otherwise electrons obey the laws of classical mechanics by travelling around the nucleus in circular orbits. Because of the quantization, the electron orbits have fixed sizes and energies. The orbits are labelled by an integer, the principle quantum number ‘n’. Bohr’s atomic model also have some drawbacks like it violated the Heisenberg’s principle which stated that the simultaneous position and velocity of an electron can’t be found but in Bohr’s model, both radius and orbits fro electrons were clearly defined. Also, it makes poor predictions regarding the spectra of larger atoms and it can’t predict the relative densities of spectral lines with inability to explain the fine structure and hyperfine structure in spectral lines. Later in 1920, Sir Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms. This conclusion arose from the disparity between an element’s atomic number (protons = electrons) and its atomic mass (usually in excess of the mass of the known protons present). For this, Sir James Chadwick was assigned the task of tracking down evidence of Rutherford’s tightly bound “proton-electron pair” or neutron. In 1930 it was discovered that Beryllium, when bombarded by alpha particles, emitted a very energetic stream of radiation. This stream was originally thought to be gamma radiation. However, further investigations into the properties of the radiation revealed contradictory results. Like gamma rays, these rays were extremely penetrating and since they were not deflected upon passing through a magnetic field, they were neutral. However, unlike gamma rays, these rays did not discharge charged electroscopes (the photoelectric effect). In 1932, Chadwick proposed that this particle was Rutherford’s neutron. In 1935, he was awarded the Nobel Prize for his discovery. Using kinematics, Chadwick was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton.
Since the development of the Atomic theory, scientists have made several changes into it and each time they concluded a new model or have come up with a slight difference from the later one. With each change, the Atomic model has become better and the mankind has got to know the fundamental components of matter. With the passing of time, The Atomic theory might get refined with new discoveries.