Atomic structure

Introduction to atomic structure: As far as the structure of the atom is concerned, the first attempt was made by John Dalton in 1803. He put forward an atomic theory which is called Dalton’s atomic theory. Following are the postulates of Dalton’s atomic theory:

1. Every matter is composed of extremely small, indivisible and discrete particles called atoms.
2. Atoms can neither be created nor be destroyed in a chemical reaction.
3. Atoms of the same element are identical in all respects.
4. Atoms of different elements have different masses and properties.
5. Two or more atoms combine in a simple whole-number ratio to form a compound atom (now called molecules).
6. The compound atoms of a compound are identical in all respects.

Fundamental particles of an atom

Electron

Electron is discovered by J.J. Thomson through a cathode ray discharge tube experiment.
The properties of cathode rays are:

1. Cathode rays are a stream of negatively charged particles.
2. They travel in a straight line.
3. They are deflected by electric and magnetic fields towards the anode.
4. They produce a heating effect when they strike then metal foil.
5. They have the property to ionize the gas through which they pass.

Later on, it was found that cathode rays are nothing but a stream of an electron. Thus an electron is a negatively charged particles having a unit negative charge and mass is equal to 1/1837 of a hydrogen atom. Mass of electron = 9.09 x 10^-28 gm.

Proton

Proton is discovered by E. Goldstein in 1886 through an anode ray discharge tube experiment.
Properties of anode rays are:

1. They consist of positively charged particles.
2. They travel in a straight line.
3. They are deflected by electric and magnetic field towards the cathode.
4. They cause a heating effect.

The observation above concludes that anode rays are positively charged protons. Thus, a proton is a positively charged particles having a unit positive charge and mass equal to that of a hydrogen atom. Mass of proton = 1.67 x 10^-24 gm.

Neutron

Neutron is discovered by James Chadwick in 1932 by bombarding α – particles on a thin sheet of beryllium. A highly penetrating neutral particle was observed called a neutron.

₂He⁴ + ₄Be⁹ → ₆C¹⁴ + ₀n¹

Thus a neutron is an electrically neutral particle having a mass nearly equal to that of a hydrogen atom. Mass of neutron=1.67 x 10^-24 gm.

Rutherford’s atomic model (alpha particle scattering experiment)

This model is put forward by Ernest Rutherford in 1911 with the help of an alpha particle scattering experiment. In this experiment, he bombarded a thin sheet of gold foil with alpha particles (positive particles equal to helium ion) which were obtained from a radioactive substance. The scattered alpha particles were observed on the surface of the circular zinc sulphide screen. Lead can absorb alpha particles. So lead plate with a slit was used to obtain a beam of alpha particles.

Observations

1. Most of the alpha particles passed through the gold foil without deflection.
2. Some of the alpha particles deflected through small angles.
3. Very few alpha particles were deflected through angles more than 90° or bounced back.

Inference

1. Most of the space inside the atom is empty.
2. There is a presence of a heavy positively charged body at the centre of the atom.
3. There is a close encounter of alpha particles with a positively charged body.

Postulates

1. An atom consists of a positively charged nucleus at which the entire mass is concentrated and its size is extremely small as compared to the size of the atom.
2. The space between the nucleus and revolving electron is empty.
3. The centrifugal force of the revolving electron is balanced by the electrostatic force of attraction between the electron and nucleus.

Limitations

1. It could not explain the stability of an atom:
   According to the law of thermodynamics, any accelerated charged particles must emit radiation. Charged particles such as an electron in motion lose energy continuously and their energy should decrease gradually. Then electron should ultimately fall into the nucleus and should collapse. But this could not happen.