notes structure of atom class 9
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Answer:
Atoms
Atoms are the building blocks of matter. It is the smallest unit of matter that is composed of three sub-atomic particles: the proton, the neutron and the electron.
Cathode ray experiment
J. J. Thomson discovered the existence of electrons.
He did this using a cathode ray tube, which is a vacuum-sealed tube with a cathode and anode on one end that created a beam of electrons travelling towards the other end of the tube.
The air inside the chamber is subjected to high voltage and electricity flows through the air from the negative electrode to the positive electrode.
The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube.
The experiment showed that the atom was not a simple, indivisible particle and contained at least one subatomic particle – the electron.
Electrons
Electrons are the negatively charged sub-atomic particles of an atom.
The mass of an electron is considered to be negligible, and its charge is -1.
The symbol for an electron is e–
Electrons are extremely small.
They are found outside the nucleus.
Thomson’s model of an atom
According to Thomson,(i) An atom consists of a positively charged sphere and the electrons are embedded in it. (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral
The first model of an atom to be put forward and taken into consideration.
He proposed a model of the atom be similar to that of a Christmas pudding/watermelon.
The red edible part of the watermelon is compared with the positive charge in the atom.
The black seeds in the watermelon are compared with the electrons which are embedded on it.
Answer:
Thomson atomic model was proposed by William Thomson in the year 1900. This model explained the description of an inner structure of the atom theoretically. It was strongly supported by Sir Joseph Thomson, who had discovered the electron earlier.
During cathode ray tube experiment, a negatively charged particle was discovered by J.J. Thomson. This experiment took place in the year 1897. Cathode ray tube is a vacuum tube. The negative particle was called an electron.
Thomson assumed that an electron is two thousand times lighter than a proton and believed that an atom is made up of thousands of electrons. In this atomic structure model, he considered atoms surrounded by a cloud having positive as well as negative charges. The demonstration of the ionization of air by X-ray was also done by him together with Rutherford. They were the first to demonstrate it. Thomson’s model of an atom is similar to a plum pudding.
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