Dalton atomic theory and its modifications.

John Dalton was an English school teacher and a researcher who gave the first scientific theory of matter based on his experiment and observations,He studied how gases combine,react,move with change in temperature and pressure and through his research he developed his idea called the Dalton atomic theory in 1803.
Postulates of Dalton atomic theory and modifications
1.All the matter is made of tiny indivisible particles known as atoms.
Modification:after the discovery of proton,elelctron and neutron the idea of indivisibility of atom was modified.
2.Atoms cannot be created or destroyed.
Modification:This statement is still applicable for ordinary chemical reactions but in nuclear reactions atoms can be created or destroyed.
3.Atoms of a given element are identical in mass,shape,size and chemical properties.
Modifications:After the discovery of isotopes this statement was rejected since isotopes are atoms of same element having same atomic number but different mass number,that means atoms of same element can have different mass
4.Atoms of different elements have different mass and chemical composition.
Modification:Atoms of different atoms can have same mass no that means same mass even though they are atoms of different elements.
5 Atoms combine in the ratio of simple whole number ratio to form compound.
Modification:It is not necessary that atoms combine in simple whole number ratio for example in organic compounds have large whole no ratios,sucrose is one of the example having formula C12H22O11
6. The relative number and kind of atoms are same in a given compound.
Modification:There are compounds which can have different composition of atoms in a given compound
You can also see the video about the same for simple expalnation!!!

Hisory of atomic theory

What is the shape of atom?and how do we know it?
well it all starts with
1.Democritus(460 B.C-370B.C)

It all starts with Democritus and his mentor luccipus,Democritus was a Greek philosopher,who lived around 400 B.C.,he actually came with the the word atomos which is a Greek word for uncut-able.
"one story to how Democritus came with the idea was that as he walked on sand he thought of how the sand used to be pebble,these pebble would have broken apart to form sand and he thought that the sand would eventually break down into smaller particle and so on until it can break no further,the particle that will be left will be the smallest indivisible particle atomos. Democritus was far ahead of his time but his ideas lacked experimental evidence and was simply based on reasoning.


2.John Dalton(Dalton atomic theory)

It wasn't until early 1800's that a man named john Dalton develop the atomic theory that would be based on his experimental evidence and observations.john Dalton was an English school teacher who studied behavior of gases under different pressure and temperature,through his research he developed his idea called the Dalton atomic theory(1803)
Dalton theory has 6 main postulates
1.All matter is composed of tiny indivisible particles called as atoms.
2.Atoms cannot be created or destroyed during a chemical reaction.
3. All atoms of the same element are identical in shape,size and chemical nature.
4. During chemical reaction atoms rearrange.
5.Compounds are formed by the combination of two or more different kind of atoms.
There are many limitations to this theory but is still is useful as the basis for the modern atomic theory.
3.J.J. Thompson(Discovery of electron(1897))

Dalton's idea of indivisibility of atoms was considered to be true for almost next 100 years until the work of j.j. Thompson,he was another English scientist who worked in late 1800's. Thompson discovered that atoms can be divided and he discovered a negative charged part of the atom called electron. Thompson worked with a device called the cathode ray tube.
Cathode ray tube is a sealed glass tube that has two electrodes separated by gas at low pressure.when high voltage was applied across the electrodes  an invisible ray known as cathode ray was produced which when strike the glass tube a glowing patch was created.Thompson discovered that if he used a magnetic field and electric field he was able to deflect these particles and the deflection showed it was negatively charged.He concluded that he these rays were negatively charged particles and named them electrons(1897).
Thompson revised the atomic theory to include newly discovered electrons and he called this theory plum pudding model.In this model atom is represented by traditional English dessert ,it is like a chocolate chip cookie where the chocolate chips represent the electrons which are embedded in rest of the atom.
4.Earnest Rutherford(discovery of nucleus(1909))

The next discovery was developed by a scientist who actually worked with Thompson,his name was earnest Rutherford is responsible for a positively charged particle of the atom that is called as proton.Rutherford fired alpha particles at very thin sheet of gold .Alpha particles are travelling at at such high speed so Rutherford and his colleagues expected the particles to be able to pass right through the gold foil,this hypothesis was based on jj Thompson model of atom.since this model assumed that electrons were distributed throughout the atom and the rest of the atom would  be positively charged portion of atom,since the positive charge was distributed throughout the atom,they did not expect there would be much interference for alpha particle.so Rutherford and his colleague believed the alpha particle would simply pass right through the atom and they would be detected on a screen on the other side,but it was not the case rather for the surprise he found that some of the particles scattered and some of the particles even deflected almost right backwards.Thompson model has to be adjusted  in order to fit the new evidence.Rutherford believed that there must be a massive particle directly in the center of the atom and this particle must be positively charged known as nucleus,they also believed that the electron must be revolving round this massive center and so atom was mostly empty space.
4.Neil Bohr(Bohr model of atom(1911))

The next discovery resulted from the research of Neil Bohr and is responsible for the transition to modern view of the atom and bohr mode is called bohr model of atom.Bohr found that electrons will eventually travel around the nucleus in discrete energy levels or electrons can only exist in these energy levels,they can never exist between these energy levels .He also found that electrons can move up an energy level if given enough energy ,he called this amount of energy a quantum of energy.A quantum of energy was just a very small amount of energy for an electron to move from one energy level to the next energy level.


5.Louis De broglie(wave particle duality)

The next discovery was from louis de broglie.He proposed that waves can act like particles and particles can act as waves,de broglie suggested that electrons should be described as waves rather than particle.This was significant because we can describe the energy of electron better by looking at wavelength rather than there mass and velocity.






6.Werner Heisenberg(Heisenberg uncertainty principle)

Werner Heisenberg proposed that one of the implication of electron behaving as wave is that we cannot know the exact position and momentum of electron at the same time.








7.Erwin Schrodinger(Schrodinger wave mechanical model of atom)

Erwin schrodinger developed a mathematical equation to show where electron are located around the nucleus of an atom .Since electron behave as waves and we cannot know precisely where they are located like Heisenberg has suggested we cannot pin point the electron to specific rings like in the bohr model .Schrodinger showed us with his wave equation that we can only know regions of probability of finding an electron and we call this an electron cloud model.




8.James Chadwick(discovery of neutron(1932))

James Chadwick was responsible for the discovery of neutron(1932).We now know that neutrons are located in the nucleus with the proton in the nucleus and electrons are present around the nucleus in an electron cloud of an atom.   


Pls check out my video on history of atomic theory for very simple and breif explanation in hindi you can also see the video in youtube