The Open Door Web Site
HOME PAGE BIOLOGY CHEMISTRY PHYSICS ELECTRONICS HISTORY HISTORY of SCI & TECH MATH STUDIES LEARN FRENCH STUDY GUIDE PHOTO GALLERY
ATOMIC and NUCLEAR ELECTRICITY and MAGNETISM MEASUREMENTS MECHANICS OPTICS PRACTICAL WORK QUESTIONS RELATIVITY THERMAL PHYSICS WAVES
ATOMIC and NUCLEAR PHYSICS
Google
Custom Search
The Rutherford Model of the Atom
In 1911 Rutherford proposed the nuclear model of atomic structure.  
He suggested that an atom consists of a central nucleus (where most of the mass of the atom is concentrated) having a positive charge, surrounded by moving electrons carrying negative charge.  
Geiger and Marsden carried out an experiment to verify his proposal.  
   
The Geiger-Marsden Alpha Particle Scattering Experiment  
The apparatus is illustrated in the diagram below.  
 
The apparatus was in an evacuated container.  
The detector was a zinc sulphide screen observed through a low power microscope.  
Each time an alpha particle hit the screen, a small flash of light was produced.  
The detector was mounted on a support such that it could be rotated to measure the angular deflection of the alpha particles as they passed through a very thin sheet of gold (something like 10-6m).  
They measured the numbers of particles deflected through various angles.  
 
It was found that most of the a particles pass through the gold undeflected, only a relatively small percentage are deflected (scattered).  
Of those that were scattered, most were deflected just a few degrees but some through quite large angles (as shown in this animation) including some that were "back-scattered" (scattering angles greater than 90°).  
Their results were considered to confirm Rutherford’s model and allowed them to estimate the size of the atom (about 10-10m) and the size of the nucleus (about 10-14m).  
   
Comparing these two figures leads us to the slightly surprising conclusion that most of the atom in empty space!  
   
Closest Approach of an Alpha Particle to a Nucleus  
For a given speed of alpha particle, the closest approach to a nucleus, rmin, will occur when the initial direction of motion of the particle is along the line joining the centres of particle and nucleus.  
   
 
In this case, at the point of closest approach, the speed of the particle is zero.  
As the particle approaches the nucleus, kinetic energy is being converted to electrical potential energy.  
We will use conservation of energy, that is, at any point, the KE lost is equal to the EPE gained.  
   
Electrical potential at a distance r from a point charge Q is given by  
 
For a nucleus of atomic number Z, the charge is Ze, where e is the magnitude of the charge on one proton (the same as the magnitude of the charge on an electron).  
The magnitude of the charge on an alpha particle is 2e  
Therefore, the energy, w, possessed by an alpha particle placed at distance, rmin, from a charge Ze is given by  
 
so we can write  
 
which means that the minimum distance is given by  
   
 
SITE MAP
WHAT'S NEW
ABOUT
PRIVACY
COPYRIGHT
SPONSORSHIP
DONATIONS
ADVERTISING
 

© The Open Door Team
2016
Any questions or
problems regarding
this site should be
addressed to
the webmaster

© David Hoult 2017

Hosted By
Web Hosting by HostCentric

 
SiteLock
 
 
Atomic and Nuclear Physics Index Page