~ ~THE THOMSON MODEL OF

THE ATOM

In 1898 ,Thompson surveyed all the evidence that the atoms

consist of charged particles .

he described an atom as a sphere of positive electricity ,

in which negative electrons are embedded .

Other people described this as the ‘plum-pudding’ model .

REASONS FOR BEING WRONG If this model of the atom is correct ,then a metal foil is a film of positive electricity containing electrons .A beam of alpha particles fired at it should pass straight through .in 1909 ,Lord Rutherford’s colleagues Geiger and Marsden tested this prediction.

GEIGER AND MARSDEN TESTED THE MODEL • They found as expected ,that a

aplha particle penetrated the goal foil

• A small fraction about (1 in 8000) of the alpha particles were deflected through large angles and even turn back on their tracks

PICTURE OF GEIGER AND MARDEN EXPERIMENT

RUTHERFORD

• Rutherford deduced that the mass and the positive charge must be concentrated in a tiny fraction of the atom ,called the nucleus

• the model of the atom which Rutherford put forward in 1911 was like the solar system .the electrons are presented in d space surrounding the nucleus .the electrons inhabit a region with a radius one hundred times thousand of times greater than that of the nucleus

• They occupy space by repelling electron of neighboring atoms .since electrons are negatively charged repulsion occur between the like charges on two electrons if they come close enough together .if another atom approaches too closely, its electrons are repelled by the electrons of the firs atom

Rutherford atomic model

H .G Mosely

• H .G Mosely suggested in 1913 that the multiple charge on the nucleus arose from the presence of proton ,which contributes the charge .since the atom are uncharged ,the number of electrons muss be the same as the number of protons *(p=e)*

• Atomic masses are greater than the mass of protons in the atom .to make up the extra mass ,the existence of neutrons postulated .these particles .these particles should have the same mass as a proton and zero charge .the search for the neutron began .Many years later it was a member of Rutherford’s team ,J Chadwick, who established the existence of the neutron in 1934

Mosely