ATOMIC MODELS

ATOMIC MODEL

Atomic theory is the scientific theory that matter is composed of particles called atoms. The definition of the word atom has changed over the years in response to scientific discoveries. Initially, it referred to a hypothetical situation in which these particles had an internal structure of their own and perhaps did not deserve to be called atoms, but renaming atoms would have been impractical by that point.

Dalton's Atomic Theory

First atomic model was developed by John Dalton. The important postulates of Daltons atomic theory are

1)Elements are made up of tiny indivisible particles called atoms.

2)Atoms can neither be created nor destroyed.

3)Atoms of a given element are identical in mass and chemical properties.

4)Atoms of different elements have masses and chemical properties.

5)Compounds are formed when atoms of different elements combined in a fixed ratio.

     Using this theory, Dalton rationalized the various laws of chemical combinations which were in existence at that time. Dalton’s theory provided a logical explanation of concepts and led the way into the new fields of further experimentations. The atomic theory was developed by further investigations and improvements. Yet the basic concept of atom that Dalton put forward remains, and we are not able to see the atoms directly by employing powerful microscopic techniques.

J . J. THOMSON’S MODEL OF ATOM

J.J. Thomson was the first to propose a detailed model of the atom. Using a cathode ray scope, Thomson determined that all matter, whatever its source, contains particles of the same kind that are much less massive than the atoms of which they form apart. They are now called electrons, although he originally called them corpuscles. The main postulates are

1) Atom has a spherical shape and it behave as positively charged.

2) The positive charge is uniformly distributed in it.

3) Electrons are embedded in the atom just like the seeds are distributed in the water melon.so this model is also known as water-melon model or plum-pudding model.

4) The advantage of the model was that it could explain the overall electrical neutrality of the atom, but fails to explain the observation of alpha particle experiment.

Rutherford Atomic Model

1)Ernest Rutherford overturned Thomson's atom model in 1911 with his well-known gold -foil experiment in which he demonstrated that the atom has a tiny, massive nucleus.

2) He beamed alpha particles through gold foil and detected them as flashes of light or scintillations on a screen.

3) The gold foil was only 0.00004 cm thick, meaning a few hundreds of atoms thick.

Observation

1)Most of the alpha particles passed through the gold foil undeflected.

2)A small fraction of the alpha particle was deflected by small angles.

3)A very few alpha particles bounced back. (nearly 180 degree).

Conclusion

1)Most of the space in an atom is empty.

2)The positive charge is concentrated in a very small volume called nucleus.

3)The volume occupied by the nucleus is negligible as compared to the total volume of the atom.

On the basis of above observation and conclusion , Rutherford proposed the nuclear model or planetary model of atom.

*The positive charge and most of the mass of the atom are concentrated in an extremely small region called nucleus.

*Electrons are revolving around the nucleus with a very high speed in circular path called orbits.

*Electrons and nucleus are held together by electrostatic force of attraction.

Drawback of Rutherford atom model

1)He could not explain the stability of atom.

2)He could not explain the electronic structure of atom.

3)He could not explain the line spectrum of hydrogen atom.

BOHR MODEL OF HYDROGEN ATOM

Neil Bohr (1913) was the first to explain quantitatively the general features of hydrogen atom structure and its spectrum. Though the theory is not the modern quantum mechanics, it can still be used to rationalize many points in the atomic structure and spectra. The postulates are

1) The electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy. These path are called orbits/stationary states and are represented as K,L,M,N shells or n= 1,2,3…. They are called principal Quantum Number.

2) The energy of electron in an orbit does not change with time. The radii of stationary states are expressed as r=n2 a0 where a0 is the Bohr radius.

3)The energy of the stationary state is given by the expression EN = -RH/n2., (where RH is the Rydberg constant and its value is -2.18 x 10^-18J.

4)Bohr's theory can also be applied to ions containing only on electron such as H+,He2+,Li3+.. and so on.

5)Angular momentum of an electron is an integral multiple of nh/2 π

mvr = nh/2 π

6)The frequency of radiation absorbed is given in the figure.

Demerits of Bohr atom model

1)It fails to explain the spectra and energy of atoms having more than one electron.

2)It fails to explain the splitting of spectral lines under the influence of electric field and magnetic field.

3)It fails to explain the stability of atoms to form molecules by chemical bond.

4)It did not consider the wave character of matter and Heisenberg uncertainty principle.