History about atomic structure from maharshi kanad - niels bohr
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Answer:
Explanation:It was Kanada who originated the idea that anu (atom) was an indestructible particle of matter. An interesting story states that this theory occurred to him while he was walking with food in his hand. As he nibbled at the food in his hand, throwing away the small particles, it occurred to him that he could not divide the food into further parts and thus the idea of a matter which cannot be divided further came into existence. He called that indivisible matter anu, i.e. atom. He also stated that anu can have two states – Absolute rest and a State of motion.
Adherents of the school of philosophy founded by Kanada, considered the atom to be indestructible, and hence eternal. They believed atoms to be minute objects invisible to the naked eye which come into being and vanish in an instant. Vaiseshikas further held that atoms of same substance combined with each other to produce dvyanuka (biatomic molecules) and tryanuka (triatomic molecules). This devinuka has the properties similar to those of the two original paramanu. Kanada also put forward the idea that atoms could be combined in various ways to produce chemical changes in presence of other factors such as heat. He gave blackening of earthen pot and ripening of fruit as example of this.
This Indian conception of the atom was developed independently and possibly prior (depending on which dates one accepts for the life of Kanada) to the development of the idea in the Greco-Roman world. Indian theories about the atom are greatly abstract and enmeshed in philosophy as they were based on logic and not on personal experience or experimentation. Thus the Indian theories lacked an empirical base, but in the words of A.L. Basham, the veteran Australian Indologist “they were brilliant imaginative explanations of the physical structure of the world, and in a large measure, agreed with the discoveries of modern physics.
Thomson Model of Atom
J.J. Thomson, the discoverer of electron in 1898 proposed the ‘raisin pudding’ model of atom. He assumed that an atom consists of a uniform sphere of positive charge with electrons embedded into it in such a way as to give it a stable configuration. In this model, the atom is visualized as a pudding or cake of positive charge with raisins (electrons) embedded into it hence, the name ‘raisin pudding’ model. Mass of the atom was considered to be evenly spread over the atom.
Rutherfod’s Model
Rutherford gave his first model in 1912. He suggested that “an atom consists of a central nucleus of small dimension within which resides the positive charge and most of the mass. Outside this nucleus are electrons to make the atom neutral”
If the electrons are assumed to be at rest in the atom, the electrons would be attracted by the nucleus and fall inside it. To remove this limitation, he gave another model which states that “the atom consists of a central nucleus surrounded by electrons which are not at rest, but revolve round the nucleus in closed paths like the planets revolving round the sun”.
An atom is neutral as the number of electrons equals the number of protons in the nucleus.
Bohr’s Model of Atom
1. The electrons move around the nucleus in one of the several fixed circular orbits called stationary energy levels. These energy levels are arranged concentrically around the nucleus, and are characterized by an integer n, the lowest level being given the number 1. The energy level corresponding to n=1, 2, 3, 4,… are also known as K, L, M, N … shells.
2. Electrons can move about only in certain orbits which have specific energies. Their movement is possible in only those orbits for which its angular momentum is an integral multiple of h/2Π or mvr=nh/2Π where ‘n’ is any integer 1, 2, 3, 4….n.
3. The energy level nearer to the nucleus has low energy, where as that farthest from it has maximum energy. An electron is said to be in ground state, when it moves in energy level having lowest energy. The ground state is the most stable state of the atom. The electron has a definite energy which is characteristic of the orbit in which it is moving. As long as the electron remains in an orbit, it does not lose energy. These orbits are hence called ‘stationary orbitals
4. Energy is emitted or absorbed when an electron moves from one level to another. Thus by absorbing one particular quantum of energy, the electron will jump from a energy level 1 to 2 or 2 to 3. It is then said to be in excited state. The quantum of energy absorbed in each case is equal to the difference in energies of the two levels. An electron cannot have an energy that would place it in between the two permissible orbits.
5. When an electron moves from a higher energy (E2) orbit to a lower energy (E1) orbit, the energy (ΔE = E2 – E1) is emitted in the form of a photon of frequency v such that ΔE=E2-E1=hv