How bohr's model can explain
(i) stability of atom
(ii) origin of line spectra
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BOHR’S MODEL OF ATOM
In order to overcome the shortcomings of Rutherford’s model, of atom Bohr (19 13) proposed a new model of atom based upon quantum theory of radiations.
Main points of this model are:
1. The electrons in an atom revolve around the nucleus only in certain selected circular orbits. These orbits are associated with definite energies and are called energy shells or energy levels. These are numbered as 1, 2, 3, 4 …… etc., or designated as
K, L, M, N . ….. , etc., shells (Fig. 5.18).
The energy of the electron is minimum in the orbit nearest to the nucleus i.e., K shell. The energy of the electron increases as it moves away from the nucleus. Electron in a particular orbit remains constant. That is why, these orbits are also called stationary states.
3. When energy from some external source is supplied to the electron, it may jump to some higher energy level by absorbing a definite amount of energy (equal to the difference in energy between th.e two energy levels). When the electron jumps back to the lower energy level it radiates the difference in energy in the form of a photon of electromagnetic radiation.
DISTRIBUTION OF ELECTRONS IN ENERGY SHELLS
As already mentioned electrons in an atom revolve around the nucleus in well defined circular orbits or energy shells. Let us now study how electrons are distributed in energy shells of an atom.
The arrangement of electrons in various energy levels of an atom is known as the electronic configuration of the atom. While writing the electronic configuration the following rules as given by Bohr and Bury are followed:
1. Electrons fill up the lowest energy levels first.
2. The maximum number of electrons in any orbit is given by the formula 2n2 where n is the number of the orbit.
For example,
For the first orbit (n = 1), the maximum number of
electrons = 2 x ( l)2 = 2
For the second orbit (n = 2), the maximum number of
electrons = 2 x (2)2 = 8
For the third orbit (n = 3), the maximum number of
electrons = 2 x (3)2 = 18
3. The outermost orbit in a stable atom cannot have more than 8 electrons even if it can accommodate more electrons according to rule 2.
4. The penultimate energy shell, i.e., the energy shell preceeding the outermost shell, cannot have more than 18 electrons.
Let us apply the above rules to write electron distribution about the nucleus in some elements.
I. Case of Hydrogen Atom
Atomic number of hydrogen is 1. There is only one electron in hydrogen atom which goes into the lowest energy shell i.e. , K shell. Thus, the electronic configuration of hydrogen atom is:
II . Case of Oxygen Atom
Now consider an atom of oxygen. Atomic number of oxygen is 8. Thus, there are eight electrons in an atom of oxygen. Two electrons are accommodated in the first shell (K shell) and the remaining six electrons are accommodated in second shell (L shell).
III. Case of Calcium Atom
Atomic number of calcium is 20. An atom of calcium contains 20 electrons. 2 electrons go in the first shell (K shell), 8 electrons go to the second shell (L shell), next 8 electrons go to the third shell (M shell) and the remaining 2 electrons go to the fourth shell (N shell). Although the third shell can accommodate a maximum of 1 8 Fig. 5.2.1 . Atomic electrons, all the 10 electrons cannot diagram’ of calcium . go into it because that would make 1 0 electrons in the outermost shell whereas the outermost shell cannot have more than 8 electrons. After the third level acquires 8 electrons, the fourth level begins to ftll. The next 2 electrons go to the fourth energy shell (N shell)
you can find your answer....
In order to overcome the shortcomings of Rutherford’s model, of atom Bohr (19 13) proposed a new model of atom based upon quantum theory of radiations.
Main points of this model are:
1. The electrons in an atom revolve around the nucleus only in certain selected circular orbits. These orbits are associated with definite energies and are called energy shells or energy levels. These are numbered as 1, 2, 3, 4 …… etc., or designated as
K, L, M, N . ….. , etc., shells (Fig. 5.18).
The energy of the electron is minimum in the orbit nearest to the nucleus i.e., K shell. The energy of the electron increases as it moves away from the nucleus. Electron in a particular orbit remains constant. That is why, these orbits are also called stationary states.
3. When energy from some external source is supplied to the electron, it may jump to some higher energy level by absorbing a definite amount of energy (equal to the difference in energy between th.e two energy levels). When the electron jumps back to the lower energy level it radiates the difference in energy in the form of a photon of electromagnetic radiation.
DISTRIBUTION OF ELECTRONS IN ENERGY SHELLS
As already mentioned electrons in an atom revolve around the nucleus in well defined circular orbits or energy shells. Let us now study how electrons are distributed in energy shells of an atom.
The arrangement of electrons in various energy levels of an atom is known as the electronic configuration of the atom. While writing the electronic configuration the following rules as given by Bohr and Bury are followed:
1. Electrons fill up the lowest energy levels first.
2. The maximum number of electrons in any orbit is given by the formula 2n2 where n is the number of the orbit.
For example,
For the first orbit (n = 1), the maximum number of
electrons = 2 x ( l)2 = 2
For the second orbit (n = 2), the maximum number of
electrons = 2 x (2)2 = 8
For the third orbit (n = 3), the maximum number of
electrons = 2 x (3)2 = 18
3. The outermost orbit in a stable atom cannot have more than 8 electrons even if it can accommodate more electrons according to rule 2.
4. The penultimate energy shell, i.e., the energy shell preceeding the outermost shell, cannot have more than 18 electrons.
Let us apply the above rules to write electron distribution about the nucleus in some elements.
I. Case of Hydrogen Atom
Atomic number of hydrogen is 1. There is only one electron in hydrogen atom which goes into the lowest energy shell i.e. , K shell. Thus, the electronic configuration of hydrogen atom is:
II . Case of Oxygen Atom
Now consider an atom of oxygen. Atomic number of oxygen is 8. Thus, there are eight electrons in an atom of oxygen. Two electrons are accommodated in the first shell (K shell) and the remaining six electrons are accommodated in second shell (L shell).
III. Case of Calcium Atom
Atomic number of calcium is 20. An atom of calcium contains 20 electrons. 2 electrons go in the first shell (K shell), 8 electrons go to the second shell (L shell), next 8 electrons go to the third shell (M shell) and the remaining 2 electrons go to the fourth shell (N shell). Although the third shell can accommodate a maximum of 1 8 Fig. 5.2.1 . Atomic electrons, all the 10 electrons cannot diagram’ of calcium . go into it because that would make 1 0 electrons in the outermost shell whereas the outermost shell cannot have more than 8 electrons. After the third level acquires 8 electrons, the fourth level begins to ftll. The next 2 electrons go to the fourth energy shell (N shell)
you can find your answer....
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