sub-atomic paticles

Cathode rays

Discharge tube

Discharge tube is also called “CROOK TUBE”. It is made of a glass tube which consists of two metallic plates. One plate is connected to positive terminal of high voltage power supply and the other to negative terminal. The plate connected to the positive terminal is called “ANODE” the other connected to negative terminal is called “CATHODE”. The tube is filled with any gas.
Experiment

In discharge tube experiment, at low pressure and at very high voltage, an electric current is passed. Due to passage of electric current, a stream of rays is passed in the tube originating from cathode. These rays are called “CATHODE RAYS”.

Properties of cathode rays

• These rays originate from cathode.

• Cathode rays travel in straight line.

• Cathode rays carry negative charge.

• Cathode rays are deflected by electric field.

• Cathode rays are deflected by magnetic field.

• These rays consist of material particles.

• The ratio e / m of these particles was 1.76 x 10⁸ col / gm.

• Cathode rays consist of “electron”.

• The rays, upon striking glass or certain other materials, cause them to glow.

Isotopes

The atoms which have the same atomic number and different mass numbers are defined as isotopes of that element.

Nuclides

Atomic species of which the proton number and nucleon number is specified.

Type of nuclides                                                                                                                                                          1. Stable nuclides existing in nature                                                                                                                       2. Unstable nuclides existing in nature                                                                                                                   3. Artificial radioactive nuclides

Radioactivity

Emission of radiation by unstable nuclei or particles spontaneously for the sake of stability of the nucleus is called radioactivity. These can penetrate and ionize gases.

Properties of alpha, beta and gamma rays

Thomson’s atomic model

Earliest theoretical description of the inner structure of atoms, proposed about 1900 by Lord Kelvin and strongly supported by Sir Joseph John Thomson, who had discovered (1897) the electron, a negatively charged part of every atom. Though several alternative models were advanced in the 1900s by Lord Kelvin and others, Thomson held that atoms are uniform spheres of positively charged matter in which electrons are embedded. Popularly known as the plum-pudding model, it had to be abandoned (1911) on both theoretical and experimental grounds in favour of the Rutherford atomic model.

Rutherford’s model

Rutherford overturned Thomson’s model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny, heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure.

Gold foil experiment

At Rutherford’s behest, Geiger and Marsden performed a series of experiments where they pointed a beam of alpha particles at a thin foil of metal and measured the scattering pattern by using a fluorescent screen. They spotted alpha particles bouncing off the metal foil in all directions, some right back at the source. This should have been impossible according to Thomson’s model; the alpha particles should have all gone straight through. Obviously, those particles had encountered an electrostatic force far greater than Thomson’s model suggested they would, which in turn implied that the atom’s positive charge was concentrated in a much tinier volume than Thomson imagined.

When Geiger and Marsden shot alpha particles at their metal foils, they noticed only a tiny fraction of the alpha particles were deflected by more than 90°. Most just flew straight through the foil. This suggested that those tiny spheres of intense positive charge were separated by vast gulfs of empty space.Most particles passed through the empty space and experienced negligible deviation, while a handful struck the nuclei of the atoms and bounced right back.

Rutherford thus rejected Thomson’s model of the atom, and instead proposed a model where the atom consisted of mostly empty space, with all its positive charge concentrated in its center in a very tiny volume, surrounded by a cloud of electrons.

Bohr model and postulates

Bohr model is based on three postulates.                                                                                                               1. Only orbits of certain radii, corresponding to certain definite energies, are permitted for the electron in a hydrogen atom.                                                                                                                                                       2. An electron in a permitted orbit has a specific energy and is in an “allowed” energy state. An electron in an allowed energy state will not radiate energy and therefore will not spiral into the nucleus.                 3. Energy is emitted or absorbed by electron only as the electron transfers from one allowed energy state to another. This energy is emitted or absorbed as a photon, E = h υ .