Gold foil experiment and Bohr's atomic model
#Alphaparticlescattering#goldfoilexperiment
#Bohrsmodel
#Rutherfordsmodel
#Rutherfordsexperiment
#Nuclearmodel
#Nuclearchemistry
To test the model Rutherford and his students performed alpha particle scattering experiment.
For the alpha particle scattering experiment a very thin gold foil was used. A circular screen of zinc sulphide was placed around the foil. Alpha particles were directed towards the foil from a radioactive source.
This experiment was carried out in a dark room.
Now let's see some Properties of alpha particles:
1)They are positively charged helium ions.
2) They have two protons and two neutrons.
They are doubly- charged.
3) Due to considerable mass and speed they could penetrate the foil.
4) They produced a flash of light (#scintillations) on striking the zinc-sulphide screen.
Why did they use foil of gold instead of other metals?
Gold is the most malleable metal and so a very thin layer of atoms can be obtained.
The gold foil used was about 4x10–⁵ cm thick.
However this thin gold film was about 1000 atoms thick.
So we can just imagine how tiny an atom is!
The #α–particles which passed through the foil would strike the screen, and produce a flash of light( scintillations)
The scientists counted the scintillations which were visible to the naked eye.
And from the location of the flash of light, the path taken by alpha particles after striking the foil could be known.
Rutherford and his students observed that most of the alpha particles passed straight through the foil.
But some particles deflected by smaller angles.
They were surprised to see that a very small fraction of them bounced back.
Rutherford said “This result was almost as incredible as if you fire a 15-inch shell at a piece of tissue paper and it comes back and hits you.
Now as per Thomsons model, the mass of the atom was distributed evenly over the entire atom due to which the positively charged field must be weak.
So it was assumed that most of the alpha particles would pass straight.
Well, it happened so.
But why did some particles deflect by smaller angles and why very few particles bounced back?
Were they repelled by some strong positive charge in the atom? Very true.
This is what Rutherford deduced from the experiment.
The atom' s positive charge is concentrated in a very small space at the centre of the atom and that most of the atom is hollow since many of the particles passed straight.
He called the centre of the atom as nucleus.
The alpha particles that went closer to the nucleus repelled due to similar charges, by smaller angles.
While particles that hit the nucleus bounced back.
Thus #Rutherfords description of the atomic model can be summarized as
(i) the atom has a positively charged centre called the nucleus which has most of the mass of the atom.
(ii) The #electrons revolve around the nucleus in circular paths.
(iii) The #nucleus is very very small as compared to the atom.
So Rutherford's alpha particle scattering experiment proved that Thomsons atomic model to be incorrect.
This experiment was a turning point in #chemistry.
Rutherford's model too had drawbacks.
Like It could not explain the #stability of atom.
We know that a particle having circular motion like that of an electron accelerates.
But an electron is a charged particle and such a charged accelerated particle radiates energy in the form of electromagnetic waves.
Due to the loss of energy the electron should eventually fall into nucleus.
Thus such atom cannot be #stable.
However this cannot be true as we know that atoms are quite stable.
In 1913, #Neils Bohr, Rutherford's student proposed some postulates to overcome the objections raised against Rutherford’s atomic model.
He proposed that electrons are restricted to concentric circular orbits or shells around the nucleus, similar to our solar system.
The orbits or shells have #quantized energies and sizes.
These orbits are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,…. The electrons do not radiate energy while revolving in these orbits.
However, An electron can jump from one energy level to another by emitting or absorbing energy.
For example when it jumps from n = 3 to n= 2 energy is emitted.
We can calculate the energy emitted or gained by the electron from the difference in the two energy levels.
Other interesting videos:
Homophones, homographs and homonymns : • Homophones, Homographs and Homonyms(Confus...
Reflection of light: • Reflection of light
Magnetism and magnetic field: • Magnetism and magnetic field lines (2020)
Refraction of light: • Refraction of light (An amazing natural ph...
Hans Oerted experiment : • Hans Oersted experiment
Word register: • Word register (Related words)
Newton's third law of motion: • Newton's third law of motion (Explained wi...
Parts of a flower: / spawcs