indivisible, atoms could be split into positively and negatively charged parts. Atoms were not just hard and solid spheres like billiard balls, but had some internal structure. On the basis of discovery of electrons and other discoveries, Thomson proposed a new atomic model in 1901. According to Thomson's atomic model, the electrically neutral atom is made up of negatively charged' electrons embedded in sphere of uniformly distributed positive charge, like currants in a plum pudding. Thomson's atomic model is the starting point in the process of understanding the structure of an atom. The next landmark in the development of modern atomic theory is Rutherford's model of an atom.
Dalton's postulate that atoms are indivisible received the first blow when an English chemist J. J. Thomson in 1897, discovered particles 1800 times lighter than the lightest atom, hydrogen. Thomson was working with cathode ray tube (Fig. 2.1). A stream of particles was emitted by cathode. These particles were found to possess negative electrical charge and were called 'electrons.' The discovery of electron was followed by many other discoveries, which indicated that atoms were notDalton's postulate that atoms are indivisible received the first blow when an English chemist J. J. Thomson in 1897, discovered particles 1800 times lighter than the lightest atom, hydrogen. Thomson was working with cathode ray tube (Fig. 2.1). A stream of particles was emitted by cathode.
Rutherford's Scattering Experiment and Atomic model :
To understand the distribution of positive and negative electrical charge inside the atom, scientists carried but experiments in which charged particles were shot through thin foils of metals like platinum and gold. The charged particles used were the a (alpha) particles having mass four times that of a hydrogen atom and positive charge twice the magnitude of the charge on an electron. The particles were studied by means of flashes of light they produced on striking a zinc sulphide screen.
A thin foil of metal was expected to contain a few layers of atoms. The motion of charged particles incident on the foil was expected to get influenced in different ways depending upon distribution of positive and negative charges and mass within the atoms. On the basis of Thomson's model of atom,.it was speculated that, on passing through the metal foil, the charged particles would get deflected through a small angle.
New Zealand born physicist Ernest Rutherford working in Cambridge, (England) carried out an experiment with a (alpha) particles and a thin gold foil. Rutherford observed that on passing through the gold foil, majority of the particles went undeflected. A small fraction of them got deflected from the original path by a small angle. However, unexpectedly, a few of the particles were scattered with large angular deviation. A still more amazing observation was that 1 out of 20,000 particles bounced
back from the metal foil with a deflection of 180° (Fig. 2.2). Rutherford acattering experiment explained the results of the scattering of particles through the gold foil by proposing a new atomic model (1911). He argued that the majority particles going undeflected indicated that an atom consists mostly of empty space. The few large angle deflections of positively charged a particles indicated that the positive charge and mass of atom were concentrated in its core which was later termed as nucleus. Rutherford proposed while putting, forth a new atomic model that the negatively charged electrons which balance the positive charge of nucleus, revolve around the nucleus like planets around the sun. Recognition that the atom has a nucleus was the most important contribution of Rutherford to understanding of atomic structure.
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Bohr's Model of Atom
According to the established laws of physics, a charged particle revolving in a circular orbit would lose energy and therefore electrons revolving around nucleus would not represent a stable state. However, under ordinary conditions most atoms are in stable state. Rutherford's atomic model could not explain the stability of atoms. By 1910 scientists had discovered that various elements emitted light on heating. The emitted light was characteristic of an element and was described as its 'emission spectrum'. It was expected that an atomic model should explain emission spectrum as well as stability of an atom. Rutherford's model could not do this. A Danish physicist, Niels Bohr proposed a new model of atom in 1913 which had provision to explain stability of an atom. Bohr's model explained Rutherford's scattering experiment as well as emission spectra. The bold postulate made by Bohr was that electrons can revolve only in certain discrete orbits around the nucleus. The electron does not lose energy when it is revolving in such discrete orbits. It can gain or lose energy by jumping from one discrete orbit to another.
After Bohr's model, many other models were proposed to account for later discoveries. Rutherford suggested in 1920 that in addition to positively charged particles, nucleus may also contain neutral particles 'neutrons'. Existence of neutrons was established later by Chadwick in 1932. The theory of quantum mechanics put forth in 1926 described atomic structure in yet another way. At the introductory stage, we shall consider the atomic structure mainly with reference >to Bohr's model (Fig. 2.3).
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