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​Welcome to Luis Diego's and Natalia's Atomic Timeline Democritus **
 * __Ancient Times (450 AD and years prior):__

Date of birth: 460 BC Date of death: 370 BC Origin: Abdera in the north of Greece Year of discovery: entire life  He hypothesized that all matter (plus space and time) is composed of tiny indestructible units, called atoms. This idea seems motivated by the question of how finely one can go on cutting up matter. While Democritus performed no experiments and had only the flimsiest evidence for postulating the existence of atoms, his theory was kept alive by the Roman poet Lucretius which survived the Dark Ages to be discovered in 1417. The atoms in Democritus theory themselves remain unchanged, but move about in space to combine in various ways to form all macroscopic objects. Early atomic theory stated that the characteristics of an object are determined by the shape of its atoms. So, for example, sweet things are made of smooth atoms, bitter things are made of sharp atoms. In this manner permanence and flux are reconciled and the field of atomic physics was born. Although Democritus' ideas were to solve a philosophical dilemma, the fact that there is some underlying, elemental substance to the Universe is a primary driver in modern physics, the search for the ultimate subatomic particle.

Leucippus Date of birth: 500 bc   Date of death: 450 BC Origin: He probably came from Miletus in Asia Minor, although Elea, Italy, and in Thrace have also been suggested. Year of discovery: // First half of 5th Century B.C. // Knowledge of Leucippus comes from the writings of Aristotle and Theophrastus. He is said to have been the teacher of Democritus and author of the Great World System and On Mind. He is also credited with being the originator of atomic theory, although it is difficult to distinguish his contributions from those of his pupil Democritus. He developed the theory of atomism, which said that the idea that everything is composed entirely of various impersihable, indivisable emlements called atoms. This was elaborated in far tgreater detail by his Pupil and Successor Democritus.

**Aristotle**

Date of birth: 384 BC Date of death: 322 BC Origin: Stagira in northern Greece

The branch of philosophy covered most by Aristotle, is metaphysics. As mentioned earlier, Aristotle's best known philosophical theory is that ideal forms or ideas, such as truth or good exist in a realm not of the material world. The word //history// comes from the ancient Greek word for knowledge attributed by Aristotle. The earth was considered round by Aristotle and Pathagoras. And the first accurate measure was in the 3rd century B.C. A geocentric was developed by Aristotle thinking that our earth is the center of the universe. This theory was "disproved" by Copernicus in the 1500's. According to Aristotle, the "golden mean" is the desirable middle ground between any two extremes. Aristotle's logic works are known as the Organon, which means instrument. Aristotle mostly used syllogism as a form of proof and drawing conclusions. However, unlike Socrates and Plato, he replaced dialectic with syllogistic logic. He made many logical inferences based on this form of arguement, syllogism.

Date of birth: January 4, 1643 Date of death: March 31, 1727 Origin: County of Lincolnshire Year of discovery: 1666
 * __1700 - 1800__**
 * Sir Isaac Newton**

Newton was educated at Trinity College, Cambridge where he lived from 1661 to 1696. During this period he produced the bulk of his work on mathematics. In 1696 he was appointed Master of the Royal Mint, and moved to London, where he resided until his death. As mathematician, Newton invented integral calculus, and jointly with Leibnitz, differential calculus. He also calculated a formula for finding the velocity of sound in a gas which was later corrected by Laplace. Newton made a huge impact on theoretical astronomy. He defined the laws of motion and universal gravitation which he used to predict precisely the motions of stars, and the planets around the sun. Using his discoveries in optics Newton constructed the first reflecting telescope. Newton found science a hodgepodge of isolated facts and laws, capable of describing some phenomena, but predicting only a few. He left it with a unified system of laws that can be applied to an enormous range of physical phenomena, and that can be used to make exact predications. Newton published his works in two books, namely "Opticks" and "Principia."

Date of birth: December 17, 1778 Date of death: May 29, 1829 Origin: Britain Year of discovery: 1800 In 1800, Davy rushed into this new field and correctly realized that the production of electricity depended on a chemical reaction taking place. His electrochemical experiments led him to propose that the tendency of one substance to react preferentially with other substances—its "affinity"—is electrical in nature. Among his many accomplishments, Davy discovered several new elements. In 1807 he electrolyzed slightly damp fused potash and then soda—substances that had previously resisted decomposition and hence were thought by some to be elements—and isolated potassium and sodium. He went on to analyze the alkaline earths, isolating magnesium, calcium, strontium, and barium. Davy's recognition that the alkalis and alkaline earths were all //oxides// challenged Lavoisier's theory that oxygen was the principle of acidity. Later, Davy determined that not all acids contain oxygen—including muriatic acid (our hydrochloric acid), which, as Davy discovered, was not "oxymuriatic acid," as Lavoisier thought. It contained only hydrogen and one other element—chlorine. In the course of his career Davy was involved in many practical projects. For example, he wrote the first text on the application of chemistry to agriculture and designed a miner's lamp that surrounded the lamp's flame with wire gauze to dissipate its heat and thus inhibit ignition of the methane gas commonly found in mines.
 * Humphry Davy**

**Daniel Bernoulli** Date of birth: February 8, 1700 Date of death: March 8, 1792 Origin: Netherlands Year of discovery: 1738 While a professor at the University of Basel, he became the first scientist outside of Great Britain to fully accept Newtonian physics. It was also here that Bernoulli performed the research on fluid behavior that would make him famous. The 1738 publication Hydrodynamica developed the prominent theories of hydrodynamics, or the movement of water. Paramount among these was the fact that, as the velocity of a fluid increases, the pressure surrounding it will decrease. Called Bernoulli's principle, this pressure drop was also shown to occur in moving air, and it is the reason boats and planes experience lift as water or air passes around them. This effect is easily shown by blowing between two pieces of paper; the drop in pressure will cause the papers to bend toward each other. Bernoulli's research marked the first attempt to explain the connection pressure and temperature have with the behavior of gas and fluids. Bernoulli's experiments with fluids caused him to devise a series of hypotheses about the nature of gases. He was certainly one of the first to formulate principles dealing with gases as groups of particles, which later became the basis for atomic theories. As groundbreaking as this work was, it was paid little attention by his peers, and subsequently it was nearly a century before the atomic theory rose again.


 * __1800 - 1875__**

**John Dalton** Date of birth: September 6, 1766 Date of death: July 27, 1844 Origin: England Year of discovery: 1803 In 1803 Dalton noted that oxygen and carbon combined to make two compounds. Of course, each had its own particular weight ratio of oxygen to carbon, but also, for the same amount of carbon, one had exactly twice as much oxygen as the other. This led him to propose the Law of Simple Multiple Proportions, which was later verified by the Swedish chemist Berzelius. In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory. The idea of atoms had been proposed much earlier. The ancient Greek philosophers had talked about atoms, but Dalton's theory was different in that it had the weight of careful chemical measurements behind it. It wasn't just a philosophical statement that there are atoms because there must be atoms. His atomic theory, stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances (remember they cannot be separated into elements by phase changes) because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.  **John Jacob Berzelius**

Date of birth: August 20, 1778 Date of death: August 7, 1848 Origin: Sweden Year of discovery: 1840 Berzelius studied medicine at [|Uppsala University] from 1796 to 1802, and from 1807 to 1832 he served as a professor of medicine and pharmacy at the [|Karolinska Institute]. He became a member of the Royal Swedish Academy of Sciences in 1808, serving from 1818 as its principal functionary, the perpetual secretary. In recognition of his growing international reputation, Berzelius was elevated to a position of nobility in 1818 on the coronation of King Charles XIV John. Berzelius was an early Swedish supporter of the new chemistry proposed a generation earlier by the renowned French chemist [|Antoine Lavoisier], and he remained a forceful exponent of enlightenment science and progressive politics even as romanticism pervaded Sweden and Europe. After initially aspiring to a career in physiological, especially animal, chemistry, he shifted his interests toward [|inorganic chemistry], the field in which he made his chief contributions. He eventually devoted considerable time to organic chemistry as well. Jacob worked on the chemical formula that is a way of expressing information about the atoms that constitute a particular chemical compound. The chemical formula identifies each element by its chemical symbol and indicates the number of atoms of each element found in discrete molecue of that compound.

Frederick Abel

Date of birth: July 17, 1827 Date of death: September 6, 1902 Origin: London Year of discovery: 1851 During a lengthy career he served as a researcher, scholar, and lecturer, and became the leading British authority on explosives. One of his most significant early discoveries was that guncotton could bechemically stabilized through thorough washing with water to remove all traces of acid and impurities. His most important work, however, came after the British government's establishment in 1888 of an Explosives Commission, dedicated in particular to the military uses of new discoveries in the field. As a member of this Commission,Abel kept in close contact with Alfred Nobel, an acquaintance from previous years whose latest invention, ballistite, was received with skepticism due tothe volatility of camphor as an ingredient. In 1889 Abel, together with JamesDewar, invented cordite, a versatile smokeless powder which purportedly improved upon ballistite through the introduction of acetone and petroleum jelly. Although Nobel contested the cordite patent, the English rights of which Abeland Dewar handed over to the government, his efforts were unsuccessful and widespread production of the propellant continued. Abel was knighted in 1891 for his invention and was made a baronet two years later.

__1875 - 1900​__

= Ernest Rutherford = Date of Birth: August 30, 1871 Date of Death: October 19, 1937 Origin: New Zealand Year of Discovery: 1910 The fourth child and second son in a family of seven sons and five daughters. His father James Rutherford, a Scottish wheelwright, emigrated to New Zealand with Ernest's grandfather and the whole family in 1842. His mother, née Martha Thompson, was an English schoolteacher, who, with her widowed mother, also went to live there in 1855. Ernest received his early education in Government schools and at the age of 16 entered Nelson Collegiate School. In 1889 he was awarded a University scholarship and he proceeded to the University of New Zealand, Wellington, where he entered​ Canterbury College.He graduated M.A. in 1893 with a double first in Mathematics and Physical Science and he continued with research work at the College for a short time, receiving the B.Sc. degree the following year. That same year, 1894, he was awarded an 1851 Exhibition Science Scholarship, enabling him to go to Trinity College, Cambridge, as a research student at the Cavendish Laboratory under J.J. Thomson. In 1897 he was awarded the B.A. Research Degree and the Coutts-Trotter Studentship of Trinity College. An opportunity came when the Macdonald Chair of Physics at McGill University, Montreal, became vacant, and in 1898 he left for Canada to take up the post. n 1907 Rutherford took the chair of physics at the University of Manchester. There along with Hans Geiger and Ernest Marsden he carried out the Geiger-Marsden experiment, which demonstrated the nuclear nature of atoms. It was his interpretation of this experiment that led him to formulate the Rutherford model of the atom—that a very small positively-charged nucleus was orbited by electrons. In 1919 he became the first person to transmute one element into another when he converted nitrogen into oxygen through the nuclear reaction 14N + α → 17O + p. In 1921, while working with Niels Bohr (who postulated that electrons moved in specific orbits), Rutherford theorized about the existence of neutrons, which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keeping the nuclei from breaking apart.

= Robert A. Millikan = Date of Birth: March 22, 1868 Date of Death: December 19, 1953 Origin: United States Year of discovery: 1910 The second son of the Reverend Silas Franklin Millikan and Mary Jane Andrews. His grandparents were of the Old New England stock which had come to America before 1750, and were pioneer settlers in the Middle West. He led a rural existence in childhood, attending the Maquoketa High School (Iowa). After working for a short time as a court reporter, he entered Oberlin College (Ohio) in 1886. During his undergraduate course his favourite subjects were Greek and mathematics; but after his graduation in 1891 he took, for two years, a teaching post in elementary physics. It was during this period that he developed his interest in the subject in which he was later to excel. In 1893, after obtaining his mastership in physics, he was appointed Fellow in Physics at Columbia University. He afterwards received his Ph.D. (1895) for research on the polarization of light emitted by incandescent surfaces - using for this purpose molten gold and silver at the U.S. Mint. As a scientist, Millikan made numerous momentous discoveries, chiefly in the fields of electricity, optics, and molecular physics. His earliest major success was the accurate determination of the charge carried by an electron, using the elegant "falling-drop method"; he also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity. Next, he verified experimentally Einstein's all-important photoelectric equation, and made the first direct photoelectric determination of Planck's constant h (1912-1915). In addition his studies of the Brownian movements in gases put an end to all opposition to the atomic and kinetic theories of matter. During 1920-1923, Millikan occupied himself with work concerning the hot-spark spectroscopy of the elements (which explored the region of the spectrum between the ultraviolet and X-radiation), thereby extending the ultraviolet spectrum downwards far beyond the then known limit. The discovery of his law of motion of a particle falling towards the earth after entering the earth's atmosphere, together with his other investigations on electrical phenomena, ultimately led him to his significant studies of cosmic radiation (particularly with ionization chambers).

= Niels Bohr = Date of Birth: October 7, 1885 Date of Death: November 18, 1962 Origin: Denmark Years of Discovery: 1911 The son of Christian Bohr, Professor of Physiology at Copenhagen University, and his wife Ellen, //née// Adler. Niels, together with his younger brother Harald (the future Professor in Mathematics), grew up in an atmosphere most favourable to the development of his genius - his father was an eminent physiologist and was largely responsible for awakening his interest in physics while still at school, his mother came from a family distinguished in the field of education. After matriculation at the Gammelholm Grammar School in 1903, he entered Copenhagen University where he came under the guidance of Professor C. Christiansen, a profoundly original and highly endowed physicist, and took his Master's degree in Physics in 1909 and his Doctor's degree in 1911. While still a student, the announcement by the Academy of Sciences in Copenhagen of a prize to be awarded for the solution of a certain scientific problem, caused him to take up an experimental and theoretical investigation of the surface tension by means of oscillating fluid jets. This work, which he carried out in his father's laboratory and for which he received the prize offered (a gold medal), was published in the Transactions of the Royal Society, 1908. In atomic physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a quantum physics-based modification of the Rutherford model, many sources combine the two, referring to the Rutherford–Bohr model. Introduced by Niels Bohr in 1913, the model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. While the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reason for the structure of the Rydberg formula, it also provided a justification for its empirical results in terms of fundamental physical constants.

__ 1915-1950 __
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= James Chadwick = Date of Birth: December 20, 1891 Date of Death: July 24, 1974 Origin: England Year of Discovery: 1932 James Chadwick was born in Cheshire, England, on 20th October, 1891, the son of John Joseph Chadwick and Anne Mary Knowles. He attended Manchester High School prior to entering Manchester University in 1908; he graduated from the Honours School of Physics in 1911 and spent the next two years under Professor (later Lord) Rutherford in the Physical Laboratory in Manchester, where he worked on various radioactivity problems, gaining his M.Sc. degree in 1913. After the war, in 1919, he returned to England to accept the Wollaston Studentship at Gonville and Caius College, Cambridge, and to resume work under Rutherford, who in the meantime had moved to the Cavendish Laboratory, Cambridge. Rutherford had succeeded that year in disintegrating atoms by bombarding nitrogen with alpha particles, with the emission of a proton. This was the first artificial nuclear transformation. In 1932, Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of //neutrons// - elementary particles devoid of any electrical charge. In contrast with the helium nuclei (alpha rays) which are charged, and therefore repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new tool in atomic disintegration need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements. Chadwick in this way prepared the way towards the fission of uranium 235 and towards the creation of the atomic bomb. For this epoch-making discovery he was awarded the Hughes Medal of the Royal Society in 1932, and subsequently the Nobel Prize for Physics in 1935.

= Erwin Schrodinger = Date of Birth: August 12, 1887 Date of Death: January 4, 1961 Origin: Austria Year of Discovery: 1926 The only child of Rudolf Schrödinger, who was married to a daughter of Alexander Bauer, his Professor of Chemistry at the Technical College of Vienna. Erwin's father came from a Bavarian family which generations before had settled in Vienna. He was a highly gifted man with a broad education. After having finished his chemistry studies, he devoted himself for years to Italian painting. After this he took up botany, which resulted in a series of papers on plant phylogeny.Schrödinger's wide interests dated from his school years at the Gymnasium, where he not only had a liking for the scientific disciplines, but also appreciated the severe logic of ancient grammar and the beauty of German poetry. In January 1926, Schrödinger published in the Annalen der Physik the paper "//Quantisierung als Eigenwertproblem//" [//tr//. Quantization as an Eigenvalue Problem] on wave mechanics and what is now known as the Schrödinger equation. In this paper he gave a "derivation" of the wave equation for time independent systems, and showed that it gave the correct energy eigenvalues for the hydrogen-like atom. This paper has been universally celebrated as one of the most important achievements of the twentieth century, and created a revolution in quantum mechanics, and indeed of all physics and chemistry.

= Werner Heisenberg = Date of Birth: December 5, 1901 Date of Death: February 1, 1976 Origin: Germany Year of Discovery: 1925 He was the son of Dr. August Heisenberg and his wife Annie Wecklein. His father later became Professor of the Middle and Modern Greek languages in the University of Munich. It was probably due to his influence that Heisenberg remarked, when the Japanese physicist Yukawa discovered the particle now known as the meson and the term "mesotron" was proposed for it, that the Greek word "mesos" has no "tr" in it, with the result that the name "mesotron" was changed to "meson". Heisenberg went to the Maximilian school at Munich until 1920, when he went to the University of Munich to study physics under Sommerfeld, Wien, Pringsheim, and Rosenthal. During the winter of 1922-1923 he went to Göttingen to study physics under Max Born, Franck, and Hilbert. In 1923 he took his Ph.D. at the University of Munich and then became Assistant to Max Born at the University of Göttingen, and in 1924 he gained the //venia legendi// at that University.From 1924 until 1925 he worked, with a Rockefeller Grant, with Niels Bohr, at the University of Copenhagen, returning for the summer of 1925 to Göttingen. In 1926 he was appointed Lecturer in Theoretical Physics at the University of Copenhagen under Niels Bohr and in 1927, when he was only 26, he was appointed Professor of Theoretical Physics at the University of Leipzig. Heisenberg's name will always be associated with his theory of quantum mechanics, published in 1925, when he was only 23 years old. For this theory and the applications of it which resulted especially in the discovery of allotropic forms of hydrogen, Heisenberg was awarded the Nobel Prize for Physics for 1932. His new theory was based only on what can be observed, that is to say, on the radiation emitted by the atom. We cannot, he said, always assign to an electron a position in space at a given time, nor follow it in its orbit, so that we cannot assume that the planetary orbits postulated by Niels Bohr actually exist. Mechanical quantities, such as position, velocity, etc. should be represented, not by ordinary numbers, but by abstract mathematical structures called "matrices" and he formulated his new theory in terms of matrix equations.

His atomic theory, stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances (remember they cannot be separated into elements by phase changes) because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.

Plum Pudding Model

The plum pudding model of the atom by J. J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus. In this model, the atom is composed of electrons (which Thomson still called "corpuscles", though G. J. Stoney had proposed that atoms of electricity be called //electrons// in 1894) surrounded by a soup of positive charge to balance the electron's negative charge, like negatively-charged "plums" surrounded by positively-charged "pudding". The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons (see below). Instead of a soup, the atom was also sometimes said to have had a cloud of positive charge.Thomson's model was compared (though not by Thomson) to a British treat called plum pudding, hence the name. It has also been called the chocolate chip cookie model or blueberry muffin model, but these mental pictures assume the particles as static, which they were not for Thomson.

Rutherford-Bohr Model



In atomic physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a quantum physics-based modification of the Rutherford model, many sources combine the two, referring to the Rutherford–Bohr model.The Bohr model is a primitive model of the hydrogen atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory. However, because of its simplicity, and its correct results for selected systems (see below for application), the Bohr model is still commonly taught to introduce students to quantum mechanics, before moving on to the more accurate but more complex valence shell atom.

**Planetary Model**

The Bohr Model is probably familar as the "planetary model" of the atom illustrated in the adjacent figure that, for example, is used as a symbol for atomic energy. In the Bohr Model the neutrons and protons (symbolized by red and blue balls in the adjacent image) occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately true in the Solar System). The adjacent image is not to scale since in the realistic case the radius of the nucleus is about 100,000 times smaller than the radius of the entire atom, and as far as we can tell electrons are point particles without a physical extent. This similarity between a planetary model and the Bohr Model of the atom ultimately arises because the attractive gravitational force in a solar system and the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons in an atom are mathematically of the same form. (The //form// is the same, but the intrinsic //strength// of the Coulomb interaction is much larger than that of the gravitational interaction; in addition, there are positive and negative electrical charges so the Coulomb interaction can be either attractive or repulsive, but gravitation is always attractive in our present Universe.)

**Electron Cloud Model**



As we all know, electrons are found to orbit around the nucleus of an atom. Each orbital in an atom is equivalent to an energy level of the electron. On absorbing a photon, an electron moves to a new quantum state by acquiring a higher level of energy. On similar lines, an electron can fall to a lower energy level by emitting a photon, thus radiating energy. The term, ‘electron cloud’ was used by the Noble Prizewinner Richard Feynman, an American physicist, in The Feynman Lectures on Physics. The model provides the means of visualizing the position of electrons in an atom. What is an electron cloud model? It is a visual model that maps the possible locations of electrons in an atom. The model is used to describe the probable locations of electrons around the atomic nucleus. The electron cloud is also defined as the region where an electron forms a three-dimensional standing wave, the one that does not move relative to the atomic nucleus. The efforts of notable scientists like Ernest Rutherford, Niels Bohr, Werner Heisenberg and others led to the formation of an electron cloud model that came to be used to estimate the positions of electrons in an atom. J.J. Thomson, a British physicist was working on cathode rays. His work led to the discovery of electrons in 1897. His revolutionary discovery prove the conventional theory wrong of atom being the indivisible unit of matter. In 1909, Rutherford brought out the fact that the positive charge and the mass of an atom is concentrated towards the center of an atom and that the electrons orbit around the atomic center. Niels Bohr, a Danish physicist modified Rutherford’s model by proposing that the orbital of electrons is restricted to predefined orbits and that they can make a transition between the orbits on absorbing or emitting energy.

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