Thomson’s model of atom(Plum pudding model) The plum pudding model of the atom by J. J.Thomson, who discovered the electron in 1897, wasproposed in 1904 before the discovery of the atomicnucleus. In this model, the atom is composed ofelectrons (which Thomson still called "corpuscles",though G. J. Stoney had proposed that atoms ofelectricity be called electrons in 1894 surrounded by asoup of positive charge to balance the electronsnegative charges, like negatively-charged "plums"surrounded by positively-charged "pudding". Theelectrons (as we know them today) were thought tobe positioned throughout the atom, but with manystructures possible for positioning multiple electrons,particularly rotating rings of electrons (see below).Instead of a soup, the atom was also sometimes saidto have had a "cloud" of positive charge.
• In this model, the electrons were free to rotate within the blobor cloud of positive substance. These orbits were stabilized inthe model by the fact that when an electron moved fartherfrom the center of the positive cloud, it felt a larger netpositive inward force, because there was more material ofopposite charge, inside its orbit ( Gausss law). In Thomsonsmodel, electrons were free to rotate in rings which werefurther stabilized by interactions between the electrons, andspectra were to be accounted for by energy differences ofdifferent ring orbits. Thomson attempted to make his modelaccount for some of the major spectral lines known for someelements, but was not notably successful at this.Still, Thomsons model (along with a similar Saturnian ringmodel for atomic electrons, also put forward in 1904 byNagaoka after James Clerk Maxwells model of Saturnsrings), were earlier harbingers of the later and moresuccessful solar-system-like Bohr model of the atom.
Contributions- Thomson devised the famous plum pudding model ofthe atom, in which electrons were compared to negativeplums embedded in a positively charged pudding. Theidea was wrong, and his successor at Cambridge,Ernest Rutherford, was soon to develop the nuclearmodel of the atom. Thomson investigated positive rays, which consist ofionized atoms, beginning in 1906. He was able to use acombination of electric and magnetic fields to separatedifferent charged atoms of elements on the basis of theircharge/mass ratios. Thomson was a great advocate of pure research, incontrast to applied research, declaring: Research inapplied science leads to reforms, research in purescience leads to revolutions, and revolutions, whetherpolitical or industrial, are exceedingly profitable things ifyou are on the winning side.
Ernst Rutherford’s model ofatom The Rutherford model or planetary model is amodel of the atom devised by Ernest Rutherford.Rutherford directed the famous Geiger-Marsdenexperiment in 1909, which suggested onRutherfords 1911 analysis that the so-called"plum pudding model" of J. J. Thomson of theatom was incorrect. Rutherfords new model forthe atom, based on the experimental results, hadthe new features of a relatively high centralcharge concentrated into a very small volume incomparison to the rest of the atom andcontaining the bulk of the atomic mass (thenucleus of the atom).
• Rutherfords model did not make any new headway inexplaining the electron-structure of the atom; in this regardRutherford merely mentioned earlier atomic models inwhich a number of tiny electrons circled the nucleus likeplanets around the sun, or a ring around a planet (such asSaturn). However, by implication, Rutherfordsconcentration of most of the atoms mass into a very smallcore made a planetary model an even more likelymetaphor than before, as such a core would contain mostof the atoms mass, in an analogous way to the Suncontaining most of the solar systems mass.
Contributions- After Rutherfords discovery, scientists started to realizethat the atom is not ultimately a single particle, but is madeup of far smaller subatomic particles. Following researchwas done to figure out the exact atomic structure which ledto Rutherford’s gold foil experiment. They eventuallydiscovered that atoms have a positively-charged nucleus(with an exact atomic number of charges) in thecenter, with a radius of about 1.2 x 10−15 meters x [AtomicMass Number]1/3. Since electrons were found to be evensmaller, this meant that the atom consists of mostly emptyspace. Later on, scientists found the expected number ofelectrons (the same as the atomic number) in an atom byusing X-rays. When an X-ray passes through anatom, some of it is scattered, while the rest passes throughthe atom. Since the X-ray loses its intensity primarily dueto scattering at electrons, by noting the rate of decrease inX-ray intensity, the number of electrons contained in anatom can be accurately estimated.
Bohr’s model of an atom In atomic physics, the Bohr model, introduced byNeil Bohr in 1913, depicts the atom as asmall, positively charged nucleus surrounded byelectrons that travel in circular orbits around thenucleus—similar in structure to the solar system, butwith electrostatic forces providing attraction, ratherthan gravity. This was an improvement on the earliercubic model (1902), the plum-pudding model(1904), the Saturnian model (1904), and theRutherford model (1911). Since the Bohr model is aquantum-physics–based modification of theRutherford model, many sources combine thetwo, referring to the Rutherford–Bohr model.
The Bohr model is a primitive model of the hydrogen atom. As atheory, it can be derived as a first-order approximation of thehydrogen atom using the broader and much more accurate quantummechanics, and thus may be considered to be an obsolete scientifictheory. However, because of its simplicity, and its correct results forselected systems (see below for application), the Bohr model is stillcommonly taught to introduce students to quantum mechanics,before moving on to the more accurate but more complex valenceshell atom. A related model was originally proposed by Arthur ErichHaas in 1910, but was rejected. The quantum theory of the periodbetween Plancks discovery of the quantum (1900) and the advent of afull-blown quantum mechanics (1925) is often referred to as the oldquantum theory.
Contribution- The Bohr model of the atom, the theory that electronstravel in discrete orbits around the atoms nucleus. The shell model of the atom, where the chemicalproperties of an element are determined by the electronsin the outermost orbit. The correspondence principle, the basic tool of Oldquantum theory. The liquid drop model of the atomic nucleus. Identified the isotope of uranium that was responsible forslow-neutron fission – 235U. Much work on the Copenhagen interpretation of quantummechanics. The principle of complementarity: that items could beseparately analyzed as having several contradictoryproperties.
Conclusion Chemistry is the science of atomic matter,especially its chemical reactions, but alsoincluding its properties, structure, composition,behavior, and changes as they relate thechemical reactions. Chemistry is centrallyconcerned with atoms and their interactionswith other atoms, and particularly with theproperties of chemical bonds. Chemistry isused by us in our daily lives though we do notrealise. Chemistry is also known as ―THECENTRAL SCIENCE‖. It is difficult to imagine your life without theknowledge of chemistry