The Thomson Model
The original belief was that the atom was a positively charged globule type thing, which had electrons sprinkled through it. This 'plum pudding' model was known as the Thomson model.
This was the widespread accepted model for the atom in the 19th century, and this wasn't changed until the 20th century.
Rutherford fired streams of alpha particles at a thin sheet of gold foil.
Behind this sheet were many detectors, which let out a small, but visible, flash of light when the alpha particle hit them. They then recorded the angle of the deflected alpha particles.
If the Thomson model was correct there would be a very small deflection of the alpha particles.
However, what was observed was that some of the alpha particles were reflected at greater then 90 degrees. This is not possible if the alpha particles are striking something that is bigger then themselves.
- More than 90% of the particles went straight through the gold foil undeflected
- Some were deflected through small angles, whiles ome were deflected through angles of greater then 90 degrees
- Some of the alpha particles were deflected back at the source (less than about 1%)
Rutherford expected these alpha particles to go straight through the gold foil, much like you would expect if you were firing cannon balls at toilet paper. The toilet paper would not effect the path of the cannon ball.
- Because a lot of the alpha particles went straight through the gold sheet, this meant that the majority of the atom was empty space.
- The deflection of the atom was due to the positively charged alpha particles being repelled, which meant that at the center of the atom there was a positive charge.
- However only a small number were reflected back at this source; this means that the center of the atom must be tiny, but very dense to cause reflection and deflection of high speed alpha particles
- Rutherford knew that the atoms had an overall neutral charge, so therefore the electrons had to be on the outside of the atom, not randomly placed within a positive sphere.
The Nuclear Atom
This gave birth to the current accept model of the atom;
- Protons and neutrons in the center of the atom, while electrons orbit this center.
As we know, electrons have a negative charge, while protons have a positive charge.
The atomic/proton number, is the number of protons in an atom. This is determines what the element is. The number of electrons will be the same as the no. of protons, providing the atom is not an ion.
The atomic mass is the number of protons and neutrons in the nucleus. Electrons have a negligible mass.
Mass number varies between atoms of the same element because of isotopes. This means that there is the same number of protons, but a different number of neutrons. The mass is an average of the isotopes of an element.