Class IX, CHEMISTRY, "Atomic Structure"
Dalton’s Atomic Theory
The important postulates of Dalton’s atomic theory are:
1. All elements are composed of atoms. Atom is too small so that it could not be divided into
further simpler components.
2. Atom cannot be destroyed or produced.
3. Atoms of an element are similar in all respects. They have same mass and properties.
4. Atoms of different elements combine in a definite simple ratio to produce compounds.
Discovery of Electron
A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum
Sir William Crooks (1895 performed experiments by passing electric current through gas in the
discharge tube at very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere
pressure, shining rays are emitted from cathode. These rays were named cathode rays. Cathode
rays are material particles as they have mass and momentum.
Properties of Cathode Rays
The properties of these particles are given below:
1. These particles are emitted from cathode surface and move in straight line.
2. The temperature of the object rises on which they fall.
3. They produce shadow of opaque object placed in their path.
4. These particles are deflected in electric and magnetic fields.
5. These particles are deflected towards positive plate of electric field.
Discovery of Proton
Gold Stein (1886) observed that in addition to the cathode rays, another type of rays were present
in the discharge tube. These rays travel in a direction opposite to cathode rays. These rays were
named positive rays. By using perforated cathode in the discharge tube the properties of these
rays can be studied. Positive rays are also composed of metered particles. The positive rays are
not emitted from anode. They are produced by the ionization of residual gas molecules in the
discharge tube. When cathode rays strike with gas molecule, electrons are removed and positive
particles are produced.
Properties of Positive Rays
1. They are deflected towards negative plate of electric field. Therefore these rays carry positive
charge. 2. The mass of positive rays is equal to the mass of the gas enclosed in the discharge tube.
3. The minimum mass of positive particles is equal to the mass of hydrogen ion (H+). These
positive ions are called Protons.
4. The charge on proton is equal to +1.602×10-