ATOMIC MODELS : If we look around us we see many objects. All these objects occupy space and have mass. This is called Matter. Numerous attempts were made to find out what this matter is made up of. Robert Boyle in 1661 found that element is the basic fundamental unit of matter.
Later on with improved discoveries, it was found that element is also made up of indivisible particles i.e. atom. This was proposed in Dalton’s Atomic Theory.
But the idea of Dalton was discarded as discovery of sub-atomic particle took place. J.J. Thomson discovered negatively charged particle called electron, E.Goldstein discovered positively charged particle called proton and J.Chadwick discovered neutral particles called neutron.
Based on this, different models of atom were proposed.
J.J. Thomson Model –
- He compared the atom to be similar to Christmas pudding. The electrons, in sphere of positive charge, were like currants (dry fruits) in spherical Christmas pudding.
- His model is also known as watermelon model i.e. the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, likes the seeds in the watermelon.
On the basis of this he proposed that an atom consists of uniform sphere in which positive charge is uniformly distributed and the electrons are embedded into it as to give the most stable electrostatic arrangement. The most stable electrostatic arrangement means that positive and negative charges are equal in magnitude which makes the atom as a whole electrically neutral.
Limitations: The similarity between seeds embedded in watermelon or currants embedded in Christmas pudding is not tenable when we have charged particles. Also, he could not explain the results carried out by other scientists with experiments.
He took a piece of radioactive substance (radium) and placed it in lead block. The block is constructed in such a way that only a narrow beam of alpha particles (the alpha particles are doubly charged helium ions) could escape through slit. The beam of high energy alpha particles was directed at thin gold foil. The purpose of using gold foil was that it is extremely malleable. A movable circular screen coated with zinc sulphide is placed around gold foil in order to detect the alpha particles. When alpha particles strike the ZnS screen, these produce flashes of light or scintillations which can be seen.
He made following 3 observations and corresponding 3 conclusions were given.
|1.||Most of the fast moving α-particles passed straight through the gold foil.||Most of the space inside the atom is empty.|
|2.||Some of the α-particles were deflected by the foil by small angles.||The positive charge of the atom occupies very little space.|
|3.||Surprisingly one out of every 12000 particles appeared to rebound.||All the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.|
On basis of 3 conclusions, the three features are proposed:
- There is a positively charged centre in an atom called the nucleus. Nearly the entire mass of an 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.
When a particle moves in circular orbit then it is said to be in uniformly accelerated motion. This is so because there is constant change in direction (despite of the fact that particle is moving with constant velocity). When a particle accelerates, then the charged particle would radiate energy. This decrease in energy may lead to a situation when electron will finally fall into the nucleus. This would make the atom highly unstable which is contradictory to the observation.
Bohr’s Model of atom:
- The objections raised against Rutherford’s Model were overcome by Neil Bohr. He proposed that
- Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
- While revolving in discrete orbits the electrons do not radiate energy. The energy is radiated only when electron leaves its orbit and moves to lower energy level or higher energy level.
- The orbits or shells are called energy levels which are designated as K, L, M, N…. or the numbers n=1, 2, 3….
Bohr and Bury suggested the distribution of electrons into different orbits of an atom . The rules are as follows:
- The maximum number of electrons which can be accommodated in a shell is given by 2n2 where n corresponds to the shell number.
- For n=1(K shell),2(1)2=2 electrons can be accommodated
- For n=2(L shell),2(2)2=8 electrons can be accommodated
- For n=3(M shell),2(3)2=18 electrons can be accommodated
- For n=4(N shell),2(4)2=32 electrons can be accommodated
- Electrons are not accommodated in a next shell until and unless the previous one is fully filled.
- The maximum electrons which can be accommodated in outermost orbit is 8.
Hence, the electronic configuration of first 20 elements is as follows:
- The Bohr atomic model made correct predictions for smaller sized atoms like hydrogen, but poor spectral predictions are obtained when larger atoms are considered.
To overcome these drawbacks, Quantum Model was proposed by Schrödinger and Heisenberg whose basis was of mathematics.
This model is the final model which we are using now.
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