Atomic Theory



Dalton’s Model of the atom. It is the basis of our understanding of the nature of matter. The following four statements comprise Dalton’s Model:

a –  All matter is made up of extremely tiny. particles. called atoms.

b-  Atoms are indivisible.

c – Atoms of the same element are alike.

d –  Atoms of different elements are different.

e –  Atoms combine in a simple ‘but fixed ratio to form molecules.

The Dalton Model developed into the Rutherford Model which describes an atom as having a nucleus surrounded by orbiting electrons.

Rutherford’s model of atom. Rutherford observed that atom has all the positive charge concentrated in a very small central region called the nucleus and that nucleus is surrounded by negatively charged particles called electrons revolving in orbits. He proved that the force of attraction between electrons and nucleus was counter balanced by the centrifugal force acting on the revolving electron. This provided stability to the atom. Rutherford explained in his experiment the large scattering of particles due to the presence of heavily charged positive nucleus. He bombarded a thin aluminum foil with a-particles and found that a-particles passing close to nucleus got defected through greater angles. This was due to very strong repulsion a-particles suffered because of high concentrated positive charge.

Drawbacks of Rutherford’s Atomic Model

a – It could not explain the distribution of electrons outside the nucleus.

b –  It could not explain the stability of atom as whole.

According to fundamental law of electric magnetic theory an accelerated charge must emit electro-magnetic energy or radiation continuously. The electrons therefore moving round the nucleus will continuously lose energy and finally they will spiral towards nucleons and fall into it when all the rotational energy got spent on radiations.

The Rutherford model developed into the Bohr Model which describes a nucleus surrounded by energy levels where electrons may be found.

Bohr’s model of the atom: Neil Bohr in 1913 observed that basic features of Rutherford’s atomic model namely presence of nucleus and revolving electrons were correct but that the electrons revolved round the nucleus in certain allowed circular orbits and that the electrons did not radiate and therefore its energy remained constant. However, the electron could jump from outer stationary orbit to inner orbit and while doing so emits radiation· equal to the difference in energies corresponding to the two orbits.

Drawbacks of Bohr’s theory:

a – On careful analysis of hydrogen spectrum it was observed that a single line in any hydrogen series· actually consisted of two or more single lines with very close wavelengths. The fine structure of hydrogen spectrum could not be explained by Bohr’s theory.

b – It could not explain relative intensity of different spectral lines.

c –  It is observed that when an electric or magnetic field is applied to the atom. each spectral line splits into several lines. The former effect known as stark effect and the later effect known as Zeeman effect could not be explained by Bohr’s theory.

d –  Bohr’s atomic model could not explain the distribution and arrangement of electrons in an atom.



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