I want the introduction notes of "structure of atom" of class 9 ncert
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Introduction to Structure of an Atom
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.
Cathode_Ray_Tube_Experiment
Apparatus of the experiment
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.
Radioactivity
Radioactivity
Radioactivity is the term for the process by which an unstable nucleus of an atom loses energy by giving out particles.
It does so by giving out particles such as alpha and beta particles.
This process is spontaneous.
An atom is unstable if the nucleus has an imbalance, meaning a difference in the protons and neutrons.
Properties of Alpha Beta and Gamma Rays
Rutherford Model
Rutherford’s experiment and observations
In this experiment, fast-moving alpha (α)-particles were made to fall on a thin gold foil. His observations were:
A major fraction of the α-particles bombarded towards the gold sheet passed through it without any deflection, and hence most of the space in an atom is empty.
Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed.
The positive charge in an atom is concentrated in a very small volume.
Very few of the α-particles were deflected back, that is only a few α-particles had nearly 180o angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.