Discovery of the atom.
From the beginning of the human civilisation people always used to think that 'what is the smallest particle?', 'And what makes different stuffs to show different properties'. Like coal burns easily and produces heat but, water do not burns. There ware lots of questions always arises in the mind of the ancient scientist.
Democritus was the Greece philosopher who used the word atomos (meaning - indivisible). According to him if we divide a stuff further and further then a time will come when it will not be further divisible and that particle will be called as atom. He was the first who gives an idea about the smallest particle. But there was no any experimental proof of its existence.
Democritus was the Greece philosopher who used the word atomos (meaning - indivisible). According to him if we divide a stuff further and further then a time will come when it will not be further divisible and that particle will be called as atom. He was the first who gives an idea about the smallest particle. But there was no any experimental proof of its existence.
John Dalton a british chemist proposed atomic theory in the beginning of 19th centuary. His theory was supported by the laws of chemical combination. One of the important fact regarding his atomic theory was atoms are individuals. In later half of 19th centuary Dimitry Mandeleve develop a periodic table in which elements were classified on the basis of similarity of their properties. This led to a natural question, "Why the atoms of different elements show similarity in their chemical properties. There was natural answer too, "If the atoms of different elements exinite similarity in their properties there must be some sort of compositional similarity of atoms if different elements. This mystery was decoded by JJ Thomson and his when they proposed electron theory of matter says electron are universal constituents of matter.
Discovery of the electron.
J J Thomson shows for the first time that atom compost of an universal particle called electron, by cathode ray experiment. He placed a discharge tube containing two metal sheets on both opposite sides. One of them were connected with positive terminal and another with negative terminal. By applying high potential difference (about 10k volts), under very low pressure (about 0.1 - 0.2 cm of hg). He observed that there were some fluorescence light came out from cathode plate and moves towards the anode plate. After doing several experiment with discharge tube, he contributed these.
What Thomson contributed after his experiment:-
- There is an universal particle is present inside the atom of every element and it is called as electron.
- Electron is a negatively charged particle which have some mass and kinetic energy.
- It always travels in straight path.
- It produces X-ray when hit hard metals.
- It affects the photographic plate.
- It shows deviation in its path under electric and magnetic field.
- Charge by mass ratio (E/M) remains same for all the gases.
After his contribution there was an another question arised that, 'If electron is negatively charged particle and present in every stuff of the universe then why they shows neutrality?' This question shows something is missing inside the atom.
Discovery of the proton.
Goldstein a German physicist repeated the Thomson's cathode ray experiment with few modification. He placed another cathode plate between the discharged tube in order to block the stream of cathode ray. That plate contain a small hole. After applying again 10k volts under very low pressure, he found that a stream of florescence starts coming out from the anode plate and moves towards the cathode plate. And Goldstein named this particle priton. After doing several experiment he found that:-
- There is an universal particle is present inside the atom of every element and it is called as proton.
- Proton is a positively charged particle which have some mass and kinetic energy.
- It always travels in straight path.
- It affects the photographic plate.
- It shows deviation in its path under electric and magnetic field.
- Charge by mass ratio (E/M) do not remains same for all the gases. But why?
- Because proton is actually ions of gases, therefore it's mass vary with gases.
After the discovery of proton there was another problem raised in order to classify the element in the periodic table. And that was, ' How the same element have different mass number ? ' Or ' If atom consist only electron and proton then why Protium have 1u mass no., Deuterium have 2u mass no and Triterium have 3u mass no. Whether they have 1 proton and 1 electron ? It means something else is missing inside the atom.
Then a new particle discovered by James Chadwick by doing another experiment. When he bombarded alpha particle on Beryllium atom, he found that it get converted into Carbon atom and a new unknown particle came out which have no charge and have mass equals to proton. It was discovered in 1932.
So, this was how scientist discovered these fundamental particle of atom but still atom were not defined. They only knew that an atom have electron, proton and neutron but they have not any idea about their location in the atom.
J. J Thomson for the first time gives a hypothetical contradiction on the structure of an atom in the year 1897. According to him, atom is just like watermelon, he supposed fleshy part of the watermelon is proton and seeds of the watermelon is just like electron. This atomic model was able to explain the neutrality of the atom and photoelectric effect, but it can't describe the electromagnetic wave theory.
Rutherford atomic model
Then another scientist named Rutherford done an experiment and gives the structure of the atom in 1911. And that experiment is called as 'Alpha particle scattering experiment'. In this experiment he took a gold foil, and put it in between photographic plate. After radiating alpha particle on the gold foil for the several times. He found that most of the alpha particle hit the photographic plate without any deviation in its path, few of them deviates a little and only 1 of 2000 deviates back on the same path. By observing these behavior of alpha particle he contradicts that:-
- The positive part (protons) of the atom is located in the center of the atom and he named it nucleus.
- The negative part of the atom (electron) present around the nucleus. And he named it extra-nuclear part.
- He contributed there is lots of gap between nucleus and extra-nuclear part if an atom.
- And the electron revolve around the nucleus.
This model was able to explain the neutrality of the atom, Photoelectric effect and this model gives the idea about the orbit of the electrons. But again this model was not able to explain the electromagnetic wave theory. According to which as the electron is revolving around the nucleus (charged particle), then it must radiate energy but Rutherford were not able to explain why electron don't loose energy. This model was very accurate.
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