Most of this section was covered in GCSE so I'll not spend long on the basics
Here are a few of the important characters.
Democritus
In a time when experiments weren't done, he first suggested that matter
was made up of tiny indivisible particles.
He invented the atom.
J.J. Thomson
Discovered that small negative particles could be removed from atoms
leaving them positive.
He discovered the electron.
The Curies
They discovered Polonium, an alpha particle emitter that Rutherford
used in his experiments.
Geiger
One of Rutherford's students. He and Marsden did most of the
experimental work that led Rutherford to his discovery of the nucleus.
Ernst Rutherford
He concluded from alpha scattering experiments, that atoms have tiny,
dense, positive nuclei.
Here's Rutherford's apparatus.
Alpha particles are fired at very thin gold foil. The number of alpha
particles which are deflected at different angles can be detected and counted.
If atoms were like plum puddings (tiny electrons in a positive sponge) then
the alpha particles (very dense, positive cannon balls) shouldn't be deflected
at all. There must be something very dense and positive inside the atoms
themselves that is scattering them.
By analyzing the scattering data Rutherford calculated the size of the nucleus
compared to the atom. Rutherford also suggested that if slower alpha particles
were used then they would be scattered more, because they spend more time next
to the repulsive nuclei. This was tried and found to be the case.
If we know the kinetic energy of the alpha particles
we can calculate the theoretical "distance of closest approach"
by putting the kinetic energy lost as it approaches the nucleus equal to
the gain in electrical P.E.
This calculation tells us that a gold nucleus is no bigger than 4 x 10-14m.
This is about 10,000 times smaller than the
atom itself
On the next page we will see a way of accurately measuring the
radii of the nuclei of different elements.
