Atomic Model Timeline

400 BC Democritus: He hypothesized that all matter (plus space and time) is composed of tiny indestructible units, called atoms. Democritus performed no experiments. 1 .All matter consists of invisible particles called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass, position, and arrangement.
->Solids are made of small, pointy atoms.
->Liquids are made of large, round atoms.
->Oils are made of very fine, small atoms that can easily slip past each other.

350 BC Aristotle did not believe in the atomic theory and he taught so otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. He did not conduct experiments or use scientific method for atomic theory. .

1800 AD It was John Dalton who determined that each chemical element is composed of a unique type of atom, and that the atoms differed by their masses. He devised a system of chemical symbols and, having ascertained the relative weights of atoms, arranged them into a table. In addition, he formulated the theory that a chemical combination of different elements occurs in simple numerical ratios by weight, which led to the development of the laws of definite and multiple proportions.
He then determined that compounds are made of molecules, and that molecules are composed of atoms in definite proportions. Thus, atoms determine the composition of matter, and compounds can be broken down into their individual elements. Dalton came with his Atomic theory as a result of his research into gases. Through deductive reasoning and experimentation, he made an interesting discovery.

1897 Joseph Thomson’s atomic theory proposed a model of atom which is known as plum pudding model or Christmas pudding or chocolate chip cookie model. Till the end of the nineteenth century the concept of atom was similar to a small solid billiard ball. In the year 1897 Joseph john Thomson totally changed the view of an atom by discovering electrons while conducting research and experiments. Thomson’s atomic theory suggested that the atom is not indivisible as it was of smaller pieces – electrons and protons.
1. An atom consists of a sphere of positive charge with negatively charged electron embedded in it,
2. The positive and the negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no overall negative or positive charge. 1913 Niels Bohr In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus. To remedy 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. 1911 Ernest Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom. Then, if he shot high velocity alpha particles (helium nuclei) at an atom then there would be very little to deflect the alpha particles. He decided to test this with a thin film of gold atoms. As expected, most alpha particles went right through the gold foil but to his amazement a few alpha particles rebounded almost directly backwards. These deflections were not consistent with Thomson's model. Rutherford was forced to discard the Plum Pudding model and reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus. He thus developed the planetary model of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus like planets around the sun. 1905 Albert Einstein He published his Special Theory of Relativity based on extensive research and experiments using the scientific method, which later led to the General Theory of Relativity, which led to his designation as “the father of modern physics.” Einstein also in 1905 proposed that light energy can be absorbed or emitted only in discrete packets called quanta, a provocative contradiction of the then-prevalent wave theory of light—and this led to Einstein’s winning of the Nobel Prize. Einstein in 1905 also explained the equivalency of mass and energy, expressed by the famous equation e=mc2. Yet these were not sufficient world-changing, revolutionary advances in physics for a single year. Einstein also in 1905 mathematically proved the existence of atoms, and thus helped revolutionize all the sciences through the use of statistics and probability

Robert Milikan 1909 Between 1908 and 1917, Robert Millikan measured the charge on an electron with an apparatus and following the scientific method. In these experiments, the atomizer from a perfume bottle was used to spray water or oil droplets into a sample chamber. Some of these droplets fell through a pinhole between two plates of an electric field, where they could be observed through a microscope. A source of x-rays was then used to ionize the air in the chamber by removing electrons from the molecules in the air. Droplets that did not capture one of these electrons fell to the bottom of the chamber due to the force of gravity. Droplets that captured one or more electrons were attracted to the positive plate at the top of the viewing chamber and either fell more slowly or rose toward the top. By carefully studying individual droplets, Millikan was able to show that the charge on a drop was always an integral multiple of a small, but finite value. When his data are converted to SI units, the charge on a drop is always some multiple of 1.59 x 10-19 C. Combining this value for the charge on a single electron with the charge to mass ratio for the electron confirms Thomson's hypothesis. The mass of an electron is at least 1000 times smaller than the lightest atom.

1900 Max Planck Planck's Quantum theory consists of the following points which were based on a series experiments which used the scientific method.
1. Light travels in wave pattern. It is an electromagnetic radiation. It travels both in medium and in vacuum. It consists of discrete particles called photons.
2. The energy of a photon is dependent on the wavelength. Substances radiate or absorb energy discontinuously in the form of small packets or bundles of discrete energy. The smallest packets are called Quanta.
3. Quantum in case of light is called a photon.

1911 Hans Geiger Rutherford with the assistance of Ernest Marsden and Hans Geiger performed a series of experiments using alpha particles. Rutherford aimed alpha particles at solid substances such as gold foil and recorded the location of the alpha particle "strikes" on a fluorescent screen as they passed through the foil. To the experimenters’ amazement, although most of the alpha particles passed unaffected through the gold foil as expected, a small number of particles were deflected at an angle, and a few ricocheted straight back. Rutherford concluded that the atom consisted of a small, dense, positively charged nucleus in the center of the atom with negatively charged electrons surrounding it. The discovery of the nucleus is considered to be Rutherford's greatest scientific work. 1932 James Chadwick He proved the existence of neutrons. James predicted the atom would have a neutron. He established that atomic number is determined by the numbers of protons in an atom. Using the scientific method and experimentation, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists. 1913 HGJ Moseley A British chemist, Henry Moseley studied under Rutherford and brilliantly developed the application of X-ray spectra to study atomic structure; Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers. In 1913, almost fifty years after Mendeleev, Henry Moseley published the results of his measurements which used the scientific method of the wavelengths of the X-ray spectral lines of a number of elements which showed that the ordering of the wavelengths of the X-ray emissions of the elements coincided with the ordering of the elements by atomic number. With the discovery of isotopes of the elements, it became apparent that atomic weight was not the significant player in the periodic law as Mendeleev, Meyers and others had proposed, but rather, the properties of the elements varied periodically with atomic number. When atoms were arranged according to increasing atomic number, the few problems with Mendeleev's periodic table had disappeared. Because of Moseley's work, the modern periodic table is based on the atomic numbers of the elements.