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Saturday, May 22, 2010

Bohr's planetary model

Bohr's planetary model:
He:
1. Developed Rutherford's atomic model
2. Did not consider the properties of electron in classical mechanics theory of Newton as Maxwell had done (by assumption of the continuous radiation of electron) must end up the destruction of the atomic system
Adopted some of Rutherford's atomic model:
1. A positively charged nucleus exists in the center of the atom
2. The number of positively charges inside the nucleus equals the number of negatively electrons charges so an atom is electrically neutral
3. While the electron orbits the nucleus, a centrifugal force appears and (compensated with the attraction force of the nucleus)
Added the following hypotheses:
1. Electrons orbit the nucleus in a rapid movement without emission or absorption of any amount of energy (losing or gaining energy)
2. Electrons orbit the nucleus only in definite allowed energy levels which means that (the electrons cannot be found at intermediate distances) OR (the distance between electron in its orbit and the nucleus constant)
3. Maximum number of energy levels in the heaviest atom = 7 (K – L – M – N – O – P – Q) expressed by a whole number called PRINCIPLE QUANTUM NUMBER (n) which = 1 for the 1st energy levels (K), = 2 for the 2nd energy levels (L), …………
4. Each energy level (orbit) has a definite amount of energy depends on the distance between the energy level and the nucleus (directly proportional with the distance)
5. The electron remains in the lower energy level in its ground state unless:
6. The electron acquires an amount of energy by heating or by electric discharge (termed QUANTUM), the electron will transfer to higher energy level agree with the absorbed energy (quantum)
7. The exited electron in the higher energy level is unstable, so it returns to its original level losing the same (quantum) of energy in the form of radiation has definite wavelength and frequency (Spectral line)
 Quantum (Q): it is the amount of energy absorbed or emitted when an electron jumps from one energy level to another OR it is the amount of energy gained or released when an electron transfers from one energy level to another
Notes:
The electron can’t move from its energy level to another unless the absorbed or emitted energy = the difference in energy between the two levels
Difference in energy between levels is not equal, so the amount of energy gained or released when an electron transfers from energy level (K) to (L) is not equal to the amount of energy gained or released when an electron transfers from (L) to (M), ………
 Advantages (adequacy) of Bohr's atomic model:
1.He explained hydrogen atom spectrum
2.He introduce the idea of quantum number to detect energy of electrons in energy level
3.He proved that electron during rotation around nucleus in ground state don't radiate energy (neither losses nor gains any energy)
4.He reconciled between Rutherford's model and Maxwell's theory
Disadvantages (inadequacy) of Bohr's atomic model:
1.He failed to explain the spectrum of any other element except hydrogen
2.He considered the electron as negative particle only (doesn't has any wave properties)
3.He postulate that it is possible to determine precisely both speed and location of an electron at same time
4.He described the atom as planer. Later it was proving that atom has 3 dimensional co – ordinates.

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