DIFFERENT ATOMIC MODELS

1. A Seminar on DIFFERENT ATOMIC MODELS Presented by: SHANTI SHARMA 4 TH SEMESTER

2. Thomson’s Atomic Model Drawbacks of Thomson’s Atomic Model Bohr atom model Drawbacks of Bohr atom model Sommerfeld’s atom model Sommerfeld’s relativistic atomic model Drawbacks of Sommerfeld’s atom model The vector atom model Conclusion References CO NTENTS

3. Thomson’s Atomic Model J. J. Thomson Electron enter into the constitution of all atoms Since the atom as a whole is electrically neutral the quantity of positive and negative charge in it must be the same.

4. Drawbacks He explained that hydrogen can give rise only to a single spectral lines. He couldn't explain the fine spectra

5. BOHR ATOM MODEL Niels Bohr He proposed the following postulates- (1)An electron cannot revolve round the nucleus in all possible orbit. It can revolve round the nucleus in those allowed orbits for which the angular momentum of the electron is an integral multiple of . Bohr’s atomic model

6. (2)An atom radiates energy only when electron jumps from a stationary orbit of higher energy to one of lower energy. If electron jumps from an initial orbit of energy to the final orbit of energy ,a photon of frequency is emitted.

7. The Bohr formulae r -e Radius of the nth permissible orbit for hydrogen The total energy of the electron in the nth orbit + Ze

8. Different spectral series of hydrogen atom according to Bohr. Lyman Balmer Paschen Brackett Pfund n=1 n=2 n=3 n=4 n=5 n=6 n=7

9. The energy level diagram The equation Can be diagrammatically represented. Then it is called The energy level diagram. ---------------------------------------------------- n=1 n=2 n=3 n=4 n=5 n=6 Lyman Balmer Paschen Brackett Pfund -13-6 -3.4 -1.5 -085

10. Drawbacks Spectrograph of high resolving showed that lines are not single. Each spectral lines actually consisted of several very close line packed together. This is called fine structure of spectral lines. Bohr theory could not explain this fine structure. Sommerfeld’s atom model Sommerfeld introduced two main modification in Bohr’s model: (1)The path of an electron around the nucleus, in general ,is an ellipse with the nucleus at one of the foci.

11. (2)The velocity of the electron moving in an elliptical orbit varies considerably at different parts of the orbit. N electron r Elliptical orbit for hydrogen atom

12. The condition that determines the allowed elliptical orbit is When and the orbit become circular has n different values

13. TOTAL ENERGY

14. Sommerfeld’s relativistic atomic model The velocity of electron in the elliptic orbits is So Sommerfeld taking into account the variation of mass with velocity. He showed that the relativistic equation describing the path of the electron is (1)

15. is called the fine structure constant The path of the electron given by equation(1) is an ellipse whose major axis precesses slowly in the plane of the ellipse about an axis through the nucleus. The total energy in the relativistic theory

16. Line is due to the transition from n=3 state to n=2 state of hydrogen atom. Fine structure of the lines

17. Drawbacks Sommerfeld’s theory was able to give an explanation of the fine structure of the spectral line of hydrogen atom. But he could not predict the correct of spectral lines.

18. The vector atom model The two distinct features of vector atom model are: The conception of spatial quantization The spinning electron hypothesis

19. Quantum no. associated with the Vector Atom Model A total quantum number n , it can take only integral values 1,2,3..etc An orbital quantum number l, which may take any integral value between 0 and (n-1) inclusively. A spin quantum number s, the magnitude of which is always ½. A total angular quantum number j, the resultant angular momentum of the electron due to both orbital and spin motions i.e vector sum of l and s.

20. Vector atom Model for Orbital Angular Momentum The orbital angular momentum for an atomic electron can be visualized in terms of a vector model where the angular momentum vector is seen as precessing about a direction in space. The diagram shows that the possible values for the "magnetic quantum number" m l for l =2 can take the values m l =-2,-1,0,1,2 or, in general, ml=-l,-l+1,……..,l-1,l

21. Vector atom Model for Total Angular Momentum When orbital angular momentum L and electron spin angular momentum S are combined to produce the total angular momentum of an atomic electron, the combination process can be visualized in terms of a vector model.

22. Conclusion Vector atom model can explain Zeeman effect, Stark effect. It can also explain the complex spectra of alkali metal like sodium. And also can explain how the orbital electrons in an atom are distributed around the nucleus.

23. ATOMIC PHYSICS - J.B. RAJAM INTRODUCTION TO ATOMIC SPECTRA -HARVEY ELLIOTT WHITE http://www.tutornext.com/lesson/vector-atom-model/1243900 http://www.chembio.uoguelph.ca/preuss/1_5_VectorModel.pdf References

24. THANK YOU