# Bohr’s Atomic Model

## Bohr’s Atomic Model

Bohr proposed a model for hydrogen atom which is also applicable for some lighter atoms in which a single electron revolves around a stationary nucleus of positive charge Ze.

Bohr’s model is based on the following postulates:

1) He postulated that an electron in an atom can move around the nucleus in certain circular stable orbits without emitting radiations.

2) Bohr found that the magnitude of the electrons, angular momentum is quantized. i.e. $$L=m{{v}_{n}}{{r}_{n}}=n\left( \frac{h}{2\pi } \right)$$ where, n = 1, 2, 3, 4, ……  each value of n corresponds to a permitted value of the orbit radius.  Where, $${{r}_{n}}$$ = Radius of nth orbit, $${{v}_{n}}$$ = Corresponding speed.

3) The radiation of energy occurs only when an electron jumps from one permitted orbit to another.

When electron jumps from higher orbit to lower energy orbit then difference of energies of these orbits i.e., $${{E}_{2}}-{{E}_{1}}$$ emits in the form of photon. But if electron goes from $${{E}_{1}}$$ to $${{E}_{2}}$$ it absorbs the same amount of energy.

Draw backs of Bohr’s Atomic Model:

1) It is valid only for one electron atoms, e.g. $$H,H{{e}^{+}},L{{i}^{+2}},N{{a}^{+1}}$$ etc.

2) Intensity of spectral lines could not be explained.

3) Nucleus was taken as stationary but it also rotates on its own axis.

4) It could not be explained the minute structure in spectrum line.

5) This does not explain the Zeeman Effect (splitting up of spectral lines in magnetic field) and Stark effect (splitting up in electric field)

6) This does not explain the doublets in the spectrum of some of the atoms like sodium ($$5890\overset{0}{\mathop{A}}\,$$ to $$5896\overset{0}{\mathop{A}}\,$$).