Bohr Atomic Theory: To overcome the limitations of Rutherford Model, Niels Bohr proposed a new model of an atom.The explanation of this theory called as Bohr Atomic Theory.He had a great contribution towards the understanding concept of atomic structure.It was on the basis of Planck’s Quantum Theory.
Niels Bohr Atomic Theory
Niels Bohr model of an atom is made up of three particles electron, proton and neutron.Electron have the negative charge; Proton has a positive charge; where is neutron has no charge. Due to the presence of an equal number of negative electrons and positive protons. The atom on the whole electrically neutral.
1.The protons and neutrons are located in a small nucleus at the centre of the atom.Due to the presence of protons nucleus is having a positive charge.
2.There is a limit to the number of electrons with the each energy level(or shell).
For example 2n2 Formula
The first energy level can hold the maximum 2 electrons.
Second energy shell can hold maximum 8 electrons.
Third energy shell can hold a maximum of 18 electrons.
Fourth energy level can hold 32 electrons.
Explanation of Bohr Atomic Theory
1. In atom, electrons revolve around the nucleus in a certain circular path called orbits.
2. Each orbit is having a definite energy. So, these are known as energy levels or energy shell. These are numbered as 1,2,3,4 or k, L, M, N…… and so on…
3.When electrons keep revolving in the same energy level.There is no change in the energy of the electron.The atom remains stable.According to Bohr atomic theory, the change in the energy of electron takes place only when it jumps from lower energy shell to higher energy shell.When it comes down from higher energy level to lower energy level it loses energy in the form of radiation(visible light, UV etc.)
4. As long as an electron present in a particular orbit, it never absorbs or loses energy. Therefore, remain constant. These energy orbits are therefore known as ground states(or stationary state).However, the electrons are not stationary, they revolve around nucleus very fast.
5.When energy is supplied to the electron it absorbs energy only in a fixed amount (as quanta). This enables to jump to higher energy state away from nucleus known as the excited state.Sinc excited state is unstable the electron made a jump to lower energy state and in doing it emit of energy is the same amount of energy.
6.The electron revolving in an orbit with the permitted angular momentum a whole number multiple of h/2π.
Achievement of Bohr Atomic Theory
Bohr Atomic theory has a many has a number of achievement to its credit.These are described as follow:
1.The theory has explained the stability of the atom. According to Bohr’s theory, the electrons present in the particular shell cannot lose the energy of its own. It can do so when jump from higher energy to lower energy level.Thus, in the ground state, the electron keep on revolving in the same circular orbit and does not come close to the nuclear portion( proton).
2.Bohr’s has helped in calculating the energy of an electron. In hydrogen atom and one electron species (He+,Li+2) it is possible to calculate the energy of hydrogen electron and also one electron species(He+,Li+2)
3.Bohr atomic theory has explained the atomic spectra of the hydrogen atom. Normally, electron tends to be in lower energy state called ground state.In case energy is made available to an electron from outside source, it absorbs energy in quanta and jumps to higher energy state called excited state. For example: in hydrogen atom only electron present in k shell in the ground state with energy E1. If it absorbs energy equal E2. It jumps to its first excited state(energy level 2).Since the excited state is unstable the electron will jump back to the ground state by losing a quantum of energy is radiation which appears in spectra.
Limitations of Bohr Atomic Theory
Bohr model was quite successful in explaining the atomic spectra of hydrogen atom and many other one electron species. It also helps to calculate the energy of an electron in different energy levels. However, suffered from certain limitation when is described as follow:
1.The theory could not explain the atomic spectra of an atom containing one or more electrons.
2. This theory fails to explain the shape of the molecule by the combination of atoms.
3. According to Bohr theory, an electron follows a fixed circular path with definite energy and thus, both his position and directions can be well defined. However, few principles stated that the path of the electron is not definite. It is probable in nature.The serious blow from certain new principle people such as de-Broglie and Heisenberg uncertainty principle.
This is whole about the article Bohr Atomic Theory.