Atomic physics

atom model

1. Bohr atom model ‘ ’An electron cannot revolve around the nucleus in all possible orbits ” -Classical theory Bohr’s postulates 1.The electron cannot revolve around the nucleus only in those allowed or permissible orbits for which the angular momentum the integral multiple of h. 2.An atom radiates energy when an electron jumps from a stationary orbit of higher energy to one of the orbit of energy and the photon is emmitted

2. By using Bohr formulae we can calculate a)radii of the stationary orbits b)total energy of the electron in the orbit SPECTRAL SERIES OF THE HYDROGEN ATOM: a) Lyman series b) Balmer series c) paschen series d) Brackett series e) Pfund series

3. CRITICAL POTENTIALS : The least energy expressed in electron volts required to excite a free neutral atom from its ground state to a higher state is called critical potential EXCITATION POTENTIALS: The energy in electron volts is required to raise an atom from its normal state into an excited state is called potential of the state IONISATION POTENTIALS: The energy required to remove an electron from a given orbit to an infinite distance from the nucleus

4. Experimental determination of critical potentials Frank and Hertz’s method This experiment shows that the energy lost by the electron in its collision with the mercury atom reappears as a quantum of energy and its wavelength Davis and Goucher’s method This experiment is used to subject to a systematic error on account of the initial velocity of emission of the electrons from the hot filament.

5. SOMMERFELD RELATIVISTIC ATOM MODEL Sommerfeld introduced two main modifications in the Bohr’s theory: 1) The path of the electron around the nucleus , in general, is an ellipse with the nucleus at one of the foci.The circular orbits of Bohr are a special case of this . 2)The velocity of the electron moving in an elliptical orbit varies considerably at different parts of the orbit.This causes relativistic variation in the mass of the moving electron.Therefore he took into account the relativistic variation of the mass of the electron with velocity .Hence this is known as THE RELATIVISTIC ATOM-MODEL.

6. Vector atom model 1) Bohr’s theory was able to explain the series of the simplest hydrogen atom. 2) The older theories were inadequate to explain new discoveries like the zeeman effect and stark effect in which the spectral lines slit under the influence of magnetic and electric fields 3) Another drawback of the Bohr model was that it could not explain how the electrons in an atom were distributed around the nucleus

7. tHE TWO DISTINCT FEATURES OF THE VECTOR ATOM MODEL ARE: 1) The conception of the spatial quantisation 2) The spinning electron hypothesis

8. There are 7 types of quantum numbers associated with the vector atom model Principal quantum number Orbital quantum number Spin quantum number Total angular quantum number Magnetic orbital quantum number Magnetic spin quantum number Magnetic total angular momentum quantum number

9. Coupling schemes L-S coupling In this type all the orbital angular momentum of the vectors c ombine to form a resultant L and independently, all their spin angular momentum vectors combine to form a resultant S. j-j coupling It is employed with the interactions between spin and orbital vectors of the each electron

10. Pauli’s exclusion principle No two electrons in an atom exist in thr same quantum state. “No two electrons in an isolated atom may have the same four quantum numbers”

11. Periodic classification of elements The periodic table is an arrangement of different elements that exist the nature, based on their chemical properties and the atomic number There are 7 periods and 8 groups in a periodic table

12. Magnetic dipole moment They are of 2 types Namely due to orbital motion of the elecctron due to spin

13. Stern and G erlach experiment This experiment is based on the behaviour f a magnetic dipole in a non-uniform magnetic field, the dipole experiences the torque that tends to align the dipole parellel to the field