Discovery of atomic structure through experiments #1

In the study of matter, the most important and basic thing is atoms and its structure. This has been discovered through a series of process and experiments by great scientists which I shall try to present below in a comprehensive manner. 


1. Plum Pudding model (or Thomson's model of atom):


The plum pudding model

The plum pudding model was a model of the atom that incorporated the recently discovered electron, and was proposed by J. J. Thomson in 1904. Thomson had discovered the electron in 1897. 

In this model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged "raisins" surrounded by positively charged "pudding". The electrons were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons.  In Thomson's model, electrons were free to rotate in rings which were further stabilized by interactions between the electrons.

2. Rutherford model of atom (Alpha particle scattering     experiment):

Rutherford overturned Thomson's model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny, heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure.

The experiment:

Ernest Rutherford conducted what is now a famous experiment where he bombarded gold foil with alpha particles (Helium nuclei).A source which undergoes alpha decay is placed in a lead box with a small hole in it.   Any of the alpha particles which hit the inside of the box are simply stopped by the box.  Only those which pass through the opening are allowed to escape, and they follow a straight line to the gold foil.


 Analysis of substances used:

 Rutherford selected a thin gold foil for the experiment because he wanted a layer as thin as possible. This gold foil was about 1000 atoms thick.

 a-particles are doubly and positively charged Helium nucleus. As they have a mass of 4 u,fast moving a-particles have a considerable amount of energy. 

Expected result:

It was expected that a-particles would be deflected by the sub-atomic particles in the gold atoms by small amounts. As the a-particles were much heavier than the protons,Rutherford did not expect to see much large deviations. 

But the experiment gave very unexpected results. In Rutherford's words - "This result was almost as incredible as if you fire a 15 inch shell at a tissue paper and it comes back and hits you."


Observation:


1. Most of the alpha particles pass straight through the gold foil.
2. Some of the alpha particles get deflected by very small amounts.
3. A very few get deflected greatly.
4. one out of every 12000 alpha particles bounced off the foil and back to the left.

Conclusion:

1. The atom is 99.99% empty space as most of the alpha particles pass straight through the gold foil without getting deflected.
2. Very few particles were deflected by small angles indicating that the positive charge of the atom occupies very little space.
3. The nucleus contains a positive charge and most of the mass of the atom in a very small space as only a very few particles deflect by 180 degree.

Rutherford's Nuclear Model of Atom:

  • There is a positively charged center in an atom called the nucleus. Nearly all the mass of the atom resides in the nucleus.
  • The electrons revolve around the nucleus in well-defined orbits.
  • The size of the nucleus is very small as compared to the size of the atom. The size of the radius of the atom is about 10^5 times than that of the radius of the nucleus.

 

  Demerits of Rutherford's Nuclear Model of Atom (Or Rutherford's Model of Atom):

  If a body is in circular motion,it will undergo acceleration. If a charged particle goes under     acceleration,it will radiate energy. slowly all the energy of the particle will be over all its structure will   collapse.This will lead to almost every form of matter to be very unstable but this is not possible as   matter is quite stable.



Some more experiments have been done to derive the presently known structure of atom which will be posted in the successive posts positively. Wish this post came handy in your study and was comprehensive.
                          To be continued >>>





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