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| The Structure of the Atom
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| Scientific investigation on the structure of atom and its subatomic particles is an area where still absorbs scientists. In the process, they have discovered many more new subatomic particles, for example positron, meson, quarks etc. We will however confine ourselves to the study to the abovementioned three fundamental subatomic particles the electron, the proton and the neutron. A basic knowledge of the nature and arrangement of these three subatomic particles is important in understanding the structure and behaviour of atoms.
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| Thomson's Model of Atom - 'Raisin Pudding' Model
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| J. J Thomson, the discoverer of the electron believed that the atom is a uniform sphere of positive charge with electrons (raisins) embedded in it.
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| This model explained the electrical neutrality of atom, but it failed to explain how these fundamental particles were arranged in the atom.
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| Rutherford's Model - Discovery of Nucleus
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| Ernest Rutherford, an energetic New Zealander working in England, was studying the properties of particles spontaneously emitted by radioactive substances. Of particular interest was a positively charged, relatively heavy emission named alpha particle. Quantities of such particles are emitted by certain radioactive elements and Rutherford's group had learnt how to control and detect them.
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| Alpha particles (a) are positively charged particles having 2 units of positive charge and 4 units of mass, i.e., these particles are doubly charged helium atoms (He2+). They are emitted by radioactive elements like radium and polonium.
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| Rutherford's Experiment
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| A stream of alpha particles (much like a stream of tiny bullets) was directed at a thin foil of gold atoms and a detector arranged to surround the sample completely, except for a small hole for entry of the particles. The foil was several thousands of atoms thick.
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| What was expected?
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| Keeping the prevailing model in mind (Thomson's 'raisin pudding') the cloud of positive electricity should offer little resistance to the passage of alpha particle even though the latter is charged. The reasoning is that alpha particles are moving at high speed and have a great amount of forward momentum (mass x velocity = momentum). Even though there would be some repulsion due to like charges, the momentum would overcome this and the particles ought to be deflected slightly or go straight through.
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| What was observed?
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Most of the alpha particles passed straight through the gold foil without any deflection from their original path
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A few alpha particles were deflected through small angles and few were deflected through large angles
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Very few alpha particles rebounded completely on hitting the gold foil and turned back in their path (just as a ball rebounds on hitting a hard wall)
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| Explanation
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Since most of the alpha particles pass straight through the gold foil without any deflection it shows there is a lot of empty space in an atom
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Some of the alpha particles are deflected through small and large angles, which shows that there is a 'centre of positive charge' in an atom, which repels the positively charged alpha particles and deflects them from the original path.
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Very few alpha particles rebound on hitting the gold foil, which shows the nucleus is very dense and hard which does not allow alpha particles to pass through it.
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| Conclusion
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Nucleus of an atom is positively charged.
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Nucleus is very dense and hard.
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Nucleus is very small compared to the size of the atom.
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The whole mass of the atom is centered at its nucleus.
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The atom of an element consists of a small, positively charged nucleus in the centre, which carries almost the entire mass of the atom.
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The electrons are revolving around the nucleus at high speed.
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The number of electrons in an atom is equal to the number of protons. Hence it is electrically neutral.
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The volume of the nucleus is negligibly small compared to the volume of the atom.
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Most of the space in the atom is empty.
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| Rutherford compared the structure of an atom to the solar system. In the solar system the Sun has the maximum mass and planets revolve around it. Similarly in an atom, the nucleus forms the main mass of atom and electrons revolve around it.
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Rutherford proposed that electrons revolve at high speed in circular orbits around the positively charged nucleus. But according to the electromagnetic theory, if a charged particle were accelerated around another charged particle then there would be a continuous radiation of energy. The loss of energy would slow down the speed of the electron and eventually the electron would fall into the nucleus. But such a collapse does not occur. Rutherford's model was unable to explain why there was no collapse.
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In order to explain the stability of an atom Neils Bohr in 1913 proposed a new theory to explain the arrangement of electrons in an atom. According to Neils Bohr, the electrons could revolve around the nucleus only in 'certain orbits' (energy levels), each orbit having a different radius. When an electron is revolving in a particular orbit or particular energy level around the nucleus, the electron does not radiate energy (lose energy) even though it has accelerated motion around the nucleus. Bohr put the following postulates about the model of the atom.
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The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells. The 'energy levels' or 'shells' or 'orbits' are represented in two ways: either by the numbers 1, 2, 3, 4, 5 and 6 or by letters K, L, M, N, O and P. The energy levels are counted from centre outwards.
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Each energy level is associated with a fixed amount of energy. The shell nearest to the nucleus has minimum energy and the shell farthest from the nucleus has maximum energy.
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There is no change in the energy of electrons as long as they keep revolving in the same energy level. But when an electron jumps from a lower energy level to a higher one, some energy is absorbed while some energy is emitted. When an electron jumps from a higher energy level to a lower one, the amount of energy absorbed or emitted is given by the difference of energies associated with the two levels. Thus, if an electron jumps from orbit 1 (energy E1) to orbit 2 (energy E2) the change in energy is given by E2 - E1. The energy change is accompanied by absorption of radiation energy of E= E2 - E1 = h where 'h' is a constant called 'Planck's constant' and  is the frequency of radiation absorbed or emitted. The value of 'h' is 6.626x10-34 J s-1. The absorption and emission of light due to electron jumps are measured by use of spectrometers.
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| This model of the atom was able to explain the stability of the atom. It also explained the phenomenon of atomic spectra and ionization of gases.
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| Although Rutherford's model was enormously successful at explaining the scattering of alpha particles, there was a problem when the experimental data were used to calculate the mass of the nucleus. From the results of the scattering experiments, it was possible to calculate the charge on the nucleus. However the mass of these protons was only about half the overall mass of the nucleus. In 1920, William Draper Harkins an American physicist suggested that the missing mass could be accounted for if the nucleus contained other particles with mass similar to that of a proton but no charge. He named this particle neutron. James Chadwick finally discovered it in 1932. He bombarded the element beryllium with alpha particles. He observed the emission of a radiation with the following properties:
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The radiation was highly penetrating.
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The radiation remained unaffected in an electric or magnetic field i.e., the radiation was neutral.
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The particles constituting the radiation had the same mass as the proton. Thus the relative mass of such a particle = 1 a.m.u and the absolute mass = 1.6 x10-24g. Because of their electrical neutrality, these particles were called neutrons.
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| The discovery of the electron, proton and neutron was the starting point of new avenues of research in science, which gave physicists an insight into the structure and nature of the atoms of matter.
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| An atom is made up of three elementary particles, namely electrons, protons and neutrons. Electrons have a negative charge, protons have a positive charge and neutrons have no charge. Neutrons are neutral. Due to the presence of equal number of negative electrons and positive protons the atom as a whole is electrically neutral. Based on the above findings, one can say that the atom has two major divisions.
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The first is the centre of an atom, called its nucleus. The protons and neutrons are located in the small nucleus at the centre of the atom. Due to the presence of protons the nucleus is positively charged.
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The second are electrons, which revolve around the nucleus in different shells (or orbits). The space around the nucleus in which the electrons revolve, determines the size of the atom.
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| Atoms of different elements are built up from different numbers of these fundamental particles. For example, if 11 protons, 11 electrons and 12 neutrons are present in an atom, it forms an atom of sodium. But if 79 protons, 79 electrons and 118 neutrons are present in an atom, then we have an atom of gold. We are now in a position of explain why the atomic mass of carbon is 12. It is now known that the carbon atom contains 6 protons and 6 neutrons each having a mass of 1
a.m.u.
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| Atomic mass of carbon = Mass of 6 protons + Mass of 6 neutrons
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| 6 x 1 + 6 x 1 = 12
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