Atomic & Nuclear Physics

Atomic Physics

Atom is the smallest part of matter which takes part in chemical reactions Atoms of the same element are similar in mass, size and characteristics. Atom consists of three fundamental particles electron, proton and neutron All the Protons and neutrons are present in the central core of atom nucleus. Electrons revolve around the nucleus.

In an atom, electrons and protons are equal in number and have equal opposite charge. Hence atom is neutral.

Properties of Fundamental Particles

Particle Discoverer Mass (Kg) Charge (Coulomb)
Elecron J. J. Thomson 9.108 x 10-31 -1.6 x 10-19
Proton Rutherford 1.672 x 10-27 +1.6 x 10-19
Neutron Chadwick 1.675 x 10-27 0

» Proton was discovered by Golastin and named by Rutherford.

Cathode Rays

If the gas pressure in a discharge tube is 10-2 to 10-3 mm of Hg and potential difference of 10 volt is applied between the electrode, then a beam of electrons emerges from the cathode which is called cathode rays. Hence cathode rays are beam of high energy electrons. Cathode is an electrode with a negative charge.

Properties of cathode rays :
1.   Cathode rays are invisible and travel in straight line.
2.   These rays carry negative charge and travel from cathode to anode.
3.   These rays emerge perpendicular to the cathode surface and are not affected by the position of anode.
4.   Cathode rays travel with very high velocity (1 / 10th the velocity of light).
5   These rays are deflected by electric and magnetic fields.
6.   TheSe rays can ionise gases.
7.   These rays heat the material on which they fall.
8.   They can produce chemical change and thus affect a photographic plate.
9.   These rays can penetrate through thin metal foils.
10.   The source of emf used in the production of cathode rays is induction coil.
11.   When they strike a target of heavy metals such as tungsten, they produce x-rays.
12.   The nature of cathode rays is independent of nature of cathode and the gas in the discharge tube.

Positive or Canal rays

If perforated cathode is used in a discharge tube, it is observed that a new type of rays are produced from anode moving towards the cathode and passed through the holes of cathode. These rays are positively charged and are called positive rays or canal rays or anode rays. These rays were discovered by Goldstein.

Properties of Canal rays :
1.   The positive rays consists of positively charged particles.
2.   These rays travel in straight line.
3.   These rays can exert pressure and thus possess kinetic energy.
4.   These rays are deflected by electric and magnetic fields.
5.   These rays are capable of producing physical and chemical changes.
6.   These rays can produce ionisation in gases.