Atoms and Molecules

Thomas suggested in 1898 that the atom is a positively charged solid sphere and electrons are embedded in it,(like nuts in a plum cake) making the sphere electrically neutral.

As Cathode rays could pass through materials of small thickness, Leonard suggested in 1903 that the atom must have a lot of empty space in it.


Rutherford did a series of experiments in 1906 to 1911 on alpha particle scattering.
Observation:

• Most of the alpha particles passed through the gold foil undeviating or with small deviation
• Some of the alpha particles deflected by large angles
• About 1 in 8000 particles were turned back by the foil

Conclusion;

• Atom must have a lot of space
• Atom must have a positively charged very heavy particle occupying a very small volume

Hence he proposed the following model of an atom,

• A positively charged nucleus of very tiny size at the center which contains almost all the mass of  the atom
• Electrons revolve around it
• Amount of positive charge on nucleus = negative charge on electrons
• Coulomb force provide the centripetal force for the electrons