Imagine you have a chocolate ice-cream cone and you want to share it with all your friends. You keep on giving out scoop after scoop until you are left with too little to even get a proper scoop. Soon enough your other friends also gather around you and ask for a share and so the process goes on.
In the end you grind the ice cream cone into a small pile and start dividing the pile into even smaller piles. Soon you will be left with just a speck of the cone. Can this process go on or will it be curtailed at some point till you get a fundamental particle which can not be divided any further?. We will discuss this and more in this section.
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John Dalton was the first to discover, "all matter is composed of small particles called atoms." According to him, the atom is a hard
solid individual particle incapable of subdivision. But it was discovered later that the atom consists of smaller particles called sub
1. Discovery of Electrons
An electron is a subatomic particle which carries a unit negative charge. It was discovered by Sir. J. J. Thomson in 1897 while working on cathode rays. These negatively charged particles coming out of the cathode ray tube were named as Electrons, since they were negatively charged.
2. Discovery of Neutrons
Proton, the positive sub atomic particle was discovered by Goldstein when working on Canal rays or the positive rays and Rutherford gave them their name.
3. Discovery of Neutrons
Neutron is a sub atomic particle, neutral in nature. It was discovered by James Chadwick in 1932.
1. Rutherford’s Model
First model of atom was given by Rutherford in 1909. Rutherford and Marsden performed their historical Alpha particle scattering experiment by directing very highly energetic alpha particles from a radioactive source against a thin gold foil provided with a circular fluorescent Zinc sulphide screen around it. A tiny flash of light was produced whenever an alpha particle struck the screen.
Based on their experiments, Rutherford called their atomic model a nuclear atom and gave forth these postulates
Rutherford’s model could not account for the electrons falling inside the nucleus a. According to the electromagnetic theory; any moving object will continuously be losing energy.
Determination of Atomic Number
Mosley brought forward the idea of atomic number. He called it the position number of a particular element in the periodic table. According to him,‘Atomic number of an element is equal to the number of protons in the nucleus of the atom of that element.’ Atomic number, therefore, was referred to as the ‘proton number' of an element.
Determination of Mass number
The total number of protons and neutrons in the nucleus of an atom is called it's mass number. Therefore, the mass number of an atom can be obtained by rounding off the experimental value of atomic mass to the nearest whole number.
With the discovery of mass number and atomic number, a new atomic model was proposed by Neil’s Bohr with the quantum theory.
When a beam of light of sufficiently high frequency is allowed to strike a metal surface in vacuum, electrons are ejected from the metal surface. The ejected electrons were termed as photo-electrons. This was proposed by Einstein and he was awarded Nobel prize in 1905 for his work on photons.
He interpreted the photoelectric effect by quantum theory.
In 1923, A. H. Compton provided one more proof to quantum theory. He was awarded the Nobel prize for his theory in 1927, which is called as the COMPTON EFFECT.
When x rays of a wavelength struck a sample of graphite, an electron was ejected and the x-rays scattered at an angle had longer wavelength.
A black body is defined as an object that absorbs all the radiations falling on it and when heated up releases this radiation. The radiations released by such a body are called black body radiations.
Max Plank proposed a theory known as Plank’s quantum theory of radiation to explain black body radiation.
According to his theory
2. Bohr’s Atomic Model
Applying Plank’s Quantum theory to the electrons revolving around the nucleus, Niels Bohr put forward his model in 1913. He retained Rutherford’s model of very small positively charged nucleus at the center which contains all the protons and neutrons present in an atom. Bohr also agreed to the concept of negatively charged electrons revolving round the nucleus in the same way as planets revolving around the sun.
Important postulates of Bohr’s theory were:
Bohr’s orbits were later changed with orbitals. Orbitals are three dimensional spaces, where the probability of finding the electron is maximum. Thus, an orbital is the most probable space in which the electron spends most of its time while in constant motion. So, the modern atomic theory or the atomic model of the present day has adopted most concepts from Bohr’s theory replacing orbits with orbitals.
Born in New Zealand, he was one among the twelve children of a farmer. His parents gave him university education with great difficulty. A brilliant student - he worked with J.J Thomson at Cambridge University to study the effect of electricity through air. He studied the spontaneous disintegration of radium.
The famous words of Rutherford on completing the alpha-rays scattering experiment were: “It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15 inch shell into a piece of tissue paper and it came back and hit you”.
He is usually credited with the first 'modern' structure of the atom, the so-called 'plum pudding' model. In it, he pictured a sphere of positive charges mixed together with an equal number of electrons. For his theoretical and experimental investigations into the electron and the conduction of electricity by gases he was awarded the 1906 Noble prize in Physics. He and his student Rutherford were the first to demonstrate the ionization of air by X-rays.
He was a great Danish Physicist who obtained his doctorate in theoretical Physics in 1911. He made a great contribution towards the understanding of atomic structure and quantum mechanics. He was awarded the Nobel prize in Physics in 1922, mainly for his work on atoms at the age of 37. During World War II, he went to USA and was one of the witnesses of the atomic explosion in New Mexico in 1945. Later he returned to Copenhagen and promoted the peaceful use of atomic energy. In 1957, he was awarded the prize for 'Atom for Peace'.
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