Structure of atom.
The Danish physicist Niels Henrik David Bohr (1885-1962) has created the theory of a structure of atom, which was deprived of the basic deficiencies of Rutherford's model of atom. Thus he had to change some habitual representations of classical physics. Bohr has formulated three postulates (sometimes the first and the second postulates are combined, then Bohr's theory of atom is reduced to two postulates):
| 1. |
The electrons in atom rotate around the kern on particular stationary orbits only. The motions on these orbits correspond to stationary states of atom, which do not vary in time without exterior actions. Rotating on stationary orbits, the electrons do not radiate energy. |
| 2. |
Allowed, discrete values of energy of an electron correspond to stationary states of atom. |
This postulate was called the quantization rule of orbits. It affirms, that when the electron is going on a stationary orbit, it should have discrete values of a moment of motion:
| m0ur = n |
|
, n = 1, 2, 3, ... |
m0 - electronic mass; u - velocity of atom;
r - radius of an orbit; h - Planck's constant;
h = 6,62 x 10-34 |
| 3. |
The radiation or uptake of energy by atom occurs only at transition of an electron from one stationary orbit to another. Thus quantum of light energy (photon) is radiated or is immersed. The energy of a quantum is equal to a difference of energies of an electron on the relevant stationary orbits: frequency of a light, energy of an electron on m and n orbits. |
The laws of classical physics say, that the transition of bodies from one state to another occurs continuously at continuous radiation or uptake of energy. The classical laws have appeared inapplicable to the phenomena, which are described by Bohr.
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Parameters of atom.
The radiuses of stationary orbits and energies of electrons are determined by dependence of energy of atom on radius of an electronic orbit. The elementary system is the hydrogen-like atom. The hydrogen-like atom is the one, in which one electron rotates on a circular orbit around of a kern, which has a charge Ze (Z - serial number of a device in periodic system).
The energy of an electron in atom consists of a kinetic energy of a motion on an orbit and potential energy in an electric field of a kern (electronic mass, travelling speed of an electron on an orbit, radius of an orbit, permittivity of vacuum of empty space):
m0 - electronic mass; u - velocity of atom;
e0 - permittivity of vacuum of empty space;
r - radius of an orbit; |
The kinetic energy is determined with the requirement, that a centripetal acceleration is created by force of a Coulomb attraction of charges. The kinetic energy of an electron is inversely proportional to radius of an orbit. A total energy:
The total energy of an electron varies with the change of a radius of an orbit the same as potential energy does. Hence, the more than radius of an orbit of an electron, which rotates around the kern, the larger is the total energy of atom.
Value r1 = 0,528 x 10-10 m is called the first Bohr's radius. For the subsequent values: rn = n2r1
n - main quantum number. It specifies an energy level, on which the atom is. The lowest state of atom is inconvertible. All atoms are in the lowest state at standard conditions. The atom can be exited, if the energy transferred to it. Thus it will be transfered to one of of higher energy states. The distance from the lowest energy level n = 1 up to the proximate level with higher energy n = 2 makes 10,1 eV = 16,2 x 10-19 J for the atom of hydrogen. This energy is the least bundle of energy, which the atom of hydrogen can immerse, when it is in a normal state.
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Deficiencies of the theory.
Bohr's theory explained the nature of a spectral series of hydrogen. It has allowed to spot theoretically the sizes of atom and energy levels of electrons for atom of hydrogen .
Deficiency of Bohr's theory is the inconsistency of its build-up. Bohr's postulates have quantum character, but the stationary electronic orbits and allowed levels of energy of atom are determined by methods of a classical mechanics and electrodynamics. Therefore Bohr's theory has appeared usable to calculations only of monovalent atoms.
Bohr's theory does not give an opportunity to calculate intensity of spectral lines, which is interlinked to probabilities of electronic transitions. The introduction of quantum rules for definition of orbits and levels of electrons was not logically justified by Bohr.
The further development of the Bohr's theory was made by German physicist Arnold Johannes Wilhelm Sommerfeld (1868-1951). He has changed circular orbits on elliptic. It has enabled to apply Bohr's theory to multielectronic atoms. However the creation of a fundamental theory of atom is bound to the creation of a quantum mechanics.
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Rutherford's
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deficiencies
Multielectronic
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