Atomic emission spectra , Bohr's atomic theory & Wave mechanical theory of the atom

From Online Sciences - December 1, 2017

On heating atoms of a pure elementsin gaseous or vapor stateto a high temperature or exposing them to a low pressure in an electrical discharge tube , they emit a radiation called emission spectrum ( line spectrum ) .

Atomic emission spectra

On examining this radiant light by a device called spectroscope , it was found that it is composed of a limited number of restricted colored lines separated by dark areas , So , it is called line spectrum , It is worth mentioning that the physicistsat that timewere not able to explain this phenomenon .

Line spectrum is a type of spectrum composed of a small number of restricted coloured lines separated by dark areas , The radiant light is named as line spectrum , because it is composed of a limited number of restricted coloured lines which are separated by dark areas .

Application : The line spectrum of hydrogen atom appears ( on examining ) as four coloured lines separated by dark areas , It was found experimentally that the spectral lines are essential characteristics for each element , because there are no two elements have the same spectral lines .

Bohrs atomic model ( 1913 )

The study of atomic spectra is considered the key which solved the puzzle of the atomic structure , That was the work of the Danish scientist Niels Bohr upon which he was rewarded the Nobel Prize in 1922 .

Bohrs Postulates

Points that agree with Rutherfords postulates

New postulates

Electrons orbit the nucleus in a rapid movement without emission or absorption of any amount of energy and the atom in this case named stable atom .

Electrons orbit the nucleus in definite allowed energy levels , They can not be found at intermediate distances , at which electron moves from an energy level to another one via a complete jumping .

Each electron in the atom has a definite amount of energy depending on the distance between its energy level and the nucleus , the energy of any level increases as its radius increases , Each energy level expressed by a whole number called the principal quantum number ( n ) .

When the electron acquires a quantity of energyknown as quantumby heating or by electric discharge , the electron jumps temporarily to a higher energy level , This is in case that the absorbed quantum of energy is equal to the difference in energies between the two levels , and the atom is known as excited atom .

Since the electron in the excited atom is unstable , it returns back to its original level with emission of the same quantum of energy ( emission spectrum ) in the form of radiant light that appears in the form of characteristic spectral line of a certain wavelength and frequency .

Quantum is the amount of energy absorbed or emitted , when an electron is transferred ( jumps ) from an energy level to another , Excited atom is an atom acquired an amount of energy by heating or by electric discharge .

The multitude of atoms absorb different amounts of energy , then radiate their energies producing spectral lines , These spectral lines correspond to the energy levels from which their electrons are transmitted back to the ground state .

The spectral line of the hydrogen atom does not represent the electron transferring from :

The quantum of energy required to transfer an electron between the different energy levels is not equal , One quantum is the energy difference between the two energy levels .

Advantages & disadvantages of Bohrs atomic model

Inadequacies of Bohrs atomic model

Principle of modern atomic theory ( modification of Bohrs model ) :

Dual nature of electron

Heisenberg uncertainty principle

Wave-mechanical theory of the atom


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