CBSE Class 9 Science Structure of Atoms Exam Notes

CBSE Class 9 Science Structure of Atoms Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.


Atoms are made up of three subatomic particles electrons, Protons and neutrons. Electron has negative charge, proton has positive charge, whereas neutron has no charge, it is neutral. Protons and neutrons are present in a small nucleus at the centre of the atom. Electrons are outside the nucleus. The atoms of different elements differ in the number of electrons, protons and neutrons.

Cathode ray

A discharge tube is long glass tube. This tube is fitted with metal electrodes on either end across which high voltage can be applied. The electrode which is connected to the negative terminal of the power source is called cathode while the electrode which is connected to the positive terminal is called anode. The tube is also connected to a vacuum pump for controlling the pressure of gas inside the discharge tube. The gas pressure is reduced to about 10–2 atmospheres and a potential difference of about 10000 volts is applied to the electrodes, an electric current flows and at the same time light is emitted by the gas. The fluorescence was caused due to the bombardment of the walls of the tube by rays emanating from cathode. These rays were called cathode rays.

 Properties of cathode rays

* Cathode rays always travel in straight line.

* Cathode rays consist of material particles and possess energy, hence they can produce mechanical effects.

* Cathode rays consist of negatively charged particles.

* Cathode rays can penetrate through thin metalic sheets.

* Cathode rays ionize the gas through which they travel.

* Cathode rays heat up the object on which they fall. When they strike an object, a part of the kinetic energy is transferred to the object resulting in a rise in temperature.

Cathode rays produce a green fluorescence on glass surface.

* When cathode rays fall on certain metals like copper, X-rays are produced. X-rays are not deflected by any electrical or magnetic fields but they pass through the opaque material and are only stopped by solid objects like bones.

* The mass of electrons is very small compared to the mass of an atom. The same type of negatively charged particles are formed even if different gases are taken in the discharge tube or different metals are used as cathode.

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