Atomic model

1. Dalton atomic model:

• Extremely small particles called atom
• Atoms combine to form a molecule.
• Atoms are indivisible.

KEYPOINT:

• Mass of electron is1/1837 times the mass of 
•  Mass of proton=1.0072amu.                                              

2. Thomas atomic model:

• In this model ,the atoms are visualised as homogeneous sphere which contain uniform distribution of positively charged particles.
• The negatively charged particles known as electrons are embedded in it like seeds in watermelon.
• The atoms are electrically neutral this implies that the total positive charge in atom is equal to total negative charge.
• According to this mo⁶del ,all the charges are assumed to be rest

DRAWBACK:

From classical electrodynamics,no stable equilibrium points exist in electrostatic configuration and hence such atom cannot to stable. 

3.Rutherford model:

EXPERIMENTAL ARRANGEMENT:

• The source of Alpha particle is kept inside the thick lead Box with the fine hole.

• The Alpha particles coming through the fine hole of lead box pass through another fine hole made on the lead screen.

• These particles are allowed to fall on a thin gold foil and it is observed that the Alpha particles passing through gold foil are scattered through different angles.

• The Movable screen which is made up of zinc sulfide is kept on the other side of the gold foil to collect the Alpha particles.
• Whenever Alpha particles strike the screen ,flash of light is observed which can be seen through a microscope.

RESULT:

• Most of the Alpha particles are undeflected through the gold foil and went straight. 
• Some of the Alpha particles are deflected through a small angle.
• A few Alpha particles are deflected through the angle more than 90 degree
• Very few Alpha particle returned back that is deflected back by180 degree.

On the basis of Rutherford experiment,

Rutherford proposed,

• The atom is most empty space.
• The nucleus is positively charged.
• The nucleus carries most of the atomic mass.
• The radius of nucleus is directly proportional to the number of nucleons.
• The volume of atom is 10^15 times the volume of nucleus.

DRAWBACK:

• Could not explain the stability of atom.
• The electron losses its energy continuously and fall into the nucleus.
• The observed spectrum should be continuous but the spectrum obtained is discrete.