Q. 55

# Why was a change in the Bohr Model of the atom required? Due to which important development (s), concept of movement of an electron in an orbit was replaced by, the concept of probability of finding the electron in an orbital? What is the name given to the changed model of an atom?

Answer :

Although Bohr’s atomic model was undoubtedly an improvisation to Rutherford’s nuclear model but it has some major drawbacks and failure when it comes to complex structures of an atom as it was too simple.

There are a few shortcomings of this model –

• It fails to explain the multiple thin and finer lines (doublet) that are found in the spectrum of hydrogen atoms which is obtained by using advanced and elaborated spectroscopic techniques.

• It is also unable to explain the spectrum of multi-electron atoms . for example even He atom which possesses 2 electrons only.

• Another fact that cannot be derived from the model is the splitting (separation) of the spectral lines on the application of a magnetic field (*Zeeman effect*) or in presence of an electric field (*Stark effect*).

• It also could not explain the cause of chemical combination (bonding) of different atoms and shapes of molecules arising from it.

• It assumes a definite knowledge about position and momentum of electrons at the same time also declining the wave-particle dual nature of an electron. ( contradicts Heisenberg’s uncertainty principle)

That is why a change and improvisation of this model was required effectively.

When de Broglie's hypothesis and Heisenberg’s uncertainty principle came to the picture the concept of an electron moving around a defined and specific orbit became insufficient in order to describe the structure of an atom.

### (i)de Broglie"s hypothesis states that every moving body posses some wave property with it which are called de Broglie waves.

### (ii)Heisenberg"s uncertainty principle states that it is impossible to determine the exact position and momentum of an electron simultaneously which also contradicts orbit concept.

### These 2 developed concepts of movement of an electron changed the scenario of the model where an electron moves in an orbit and later was replaced by the concept of orbital which considered a space around nucleus where the probability of finding electron is maximum.

### • For the wave-like properties of the electron, it cannot be considered only as a particle and obtaining a definite position and velocity was also impossible.

### • Hence electron now in the developed concepts was considered as electromagnetic energies which have quantized energy at certain energy levels which is the direct application of its wave nature.

• Again, Both the exact position and exact velocity of an electron(or any particle) in an atom cannot be determined simultaneously(at a certain time: *Heisenberg’s uncertainty principle*). Therefore the determination of the path of movement of an electron in an atom is almost impossible or it cannot be known accurately. That is why, , the concept of probability of finding the electron at different points in an atom comes to the picture.

The name given to the changed model of atom is “Quantum Mechanical Model of Atom” and was developed by the wave equation given by *Erwin Schrödinger**.*

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