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Introduction |
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In the nineteenth century scientists saw enough evidence to prove that each element has its distinctive atom. But all tem atoms contain identical electrons. In spite of the fact that electrons carry negative charge, atoms as whole were found to be neutral. |
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Thomson's Model |
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According to this model, an atom can be considered as a sphere of uniformly distributed positive charge in which there are electrons distributed symmetrically. |
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Rutherford's Experiment |
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A radioactive source S emitting a particles was collimated into a fine beam and made to fall on a thin gold foil. The a particles scattered in all directions. |
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The Electric Potential Energy |
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The electric potential energy of the alpha particle at the distance of closest approach from the nucleus. |
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Rutherford's Atom Model (Main Features) |
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Every atom consists of a nucleus containing the entire +ve charge. The whole mass of atom is concentrated at this core. |
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Bohr's Theory of Hydrogen Atom |
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Niels Bohr suggested that the problem about hydrogen spectrum can be solved if we can make some assumptions. |
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Excitation and Ionisation Potential |
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An electron revolving in a stationary orbit of an atom absorbs some energy the electron may jump over to an orbit of higher energy. This process is called excitation and the atom is said to be in the excited state. The energy absorbed to move from one orbit to the other is called excitation potential. |
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Emission and Absorption Spectra |
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One can understand atomic spectra if one knows the concept of atomic energy levels. The movements of electrons from one level to another causes the spectra. The work of a spectoscopicst is to find the energy levels of an atom from the measured values of the wavelengths of the spectral lines emitted by the atoms. To analyze the spectra emitted by the lighter atoms is easy and that by heavier atoms is difficult. |
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Moseley's Law |
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Moseley had done experiments on the characteristic X-rays and this led to the development of the concept of atomic number. |
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Atomic Number |
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The number of protons or the number of electrons in an atom in its normal state is called the atomic number and is denoted by Z. The nucleus of every element contains Z protons and some number of neutrons. |
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Spontaneous and Stimulated Radiation |
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When an atom jumps from a higher energy stated to a lower energy state it emits light in the form of photons. In any source of light, the light that is emitted is incoherent, i.e., different photons have different phases and different wavelengths. |
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Atomic Nucleus |
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It is the central core of the atom where the entire +ve charge and mass is concentrated. |
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Relation Between Atomic Mass Unit and MeV (Mass Energy Relation) |
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The unit in which atomic and nuclear masses are measured is called atomic mass unit (a.m.u). |
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Isotopes, Isobars and Isotones |
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These are the elements having same atomic number but different mass number. They have the same atomic number because the number of protons inside their nuclei remains the same. The difference in their mass number is due to the difference in their number of neutrons. |
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Mass Energy Relation |
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Einstein derived a formula given the relation between mass and energy as E = mc2. |
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Binding Energy and Mass Defect |
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An atomic nucleus is a stable structure. The nucleus is bound by very strong short range forces called nuclear forces. Certain amount of work has to be done to separate the nucleons to such a distance that there is no interaction. This work done therefore measures binding energy of the nucleus. |
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Binding Energy |
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Average binding energy per nucleon is the total binding energy divided by the mass number of the nucleus. |
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Nature of Nuclear Forces |
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Physicists have categorized all forces occurring in nature under gravitational, electromagnetic, strong nuclear and weak nuclear forces. |
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Radioactivity |
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It is a spontaneous emission or disintegration of an unstable nucleus resulting in certain radiations. The elements exhibiting this phenomena are called radioactive elements. e.g., radium, thorium, actinium, polonium etc. |
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Laws of Radioactive Disintegration |
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The number of atoms disintegrated per second at any instant is directly proportional to the number of radioactive atoms actually present in the sample at that instant. |
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Nuclear Fission |
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Neutrons are considered as the best projectile particles to hit the target (uranium) nucleus. The uranium may absorb these neutrons and become 92U236which later splits into two smaller nuclei. In addition two neutrons are released and energy is also released. This process is called a Nuclear fission. |
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Nuclear Reactor |
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A nuclear reactor is an installation where a self-sustaining nuclear fission takes place in a controlled manner and energy released is used for constructive purposes. |
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Nuclear Fusion |
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Nuclear fusion is the phenomenon of fusing two or more lighter nuclei to form a single heavy nucleus. |
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Summary |
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Dalton had postulated that matter is made up of atoms, which are indivisible. Thomson was the first to suggest a structure for an atom. According to him, an atom is a positively charged sphere of radius=10-10 m in which the mass and the positive charge of the atom are uniformly distributed. Inside the sphere, electrons carrying equal negative charge are embedded like seeds in a watermelon. This model failed, as it could not explain the origin of spectral series of hydrogen atom. |
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Conclusion |
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Rutherford's alpha particle scattering experiment explained the structure of atom and the size of nucleus. |
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Numerical -1 |
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What is the distance of closest approach to the nucleus of an alpha particle which undergoes scattering by 180o in the Gelger- Marsden experiment? |
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Numerical -2 |
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Determine the radius of the first orbit of the hydrogen atom. What would be the velocity and frequency of the electron in the first orbit? Given: h = 6.62 x 10-34 Js, m = 9.1 x 10-31 kg, e = 1.6 x 10-19 C, k = 9 x 109 N m2 C-2. |
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Numerical -3 |
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Calculate the value of constant. |
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Numerical -4 |
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Calculate the ionisation potential for a lithium atom |
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Numerical -5 |
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The second member of Lyman series in hydrogen spectrum. |
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Numerical -6 |
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Using the Rydberg formula, calculate the wavelengths of the first four spectral lines in the Balmer series of the hydrogen spectrum. |
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Numerical -7 |
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Calculate the packing fraction of a particle |
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Numerical -8 |
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A nucleus of UX1 has a half life of 24.1 days. How long a sample of UX1 will take to change 90% of it to UX2? |
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Numerical -9 |
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The half life of radon is 3.8 days. Calculate how much radon will be left out of 10.24 milligram after 19 days? |
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Numerical -10 |
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Determine the half life of a radioactive material. |
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Numerical -11 |
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If 200 MeV energy is released in the fission of a sample nucleus of 92U235, how many fissions must occur per second to produce a power of 1 kW? |
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Numerical -12 |
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The half-life of 92U238 against alpha decay is 4.5 x 109 years. How many disintegrations per second occur in 1 g of 92U238? |
