- Parent Category: 06 Quantum Physics
- Category: 02 Nuclear
- Created: Thursday, 17 December 2015 16:37
- Last Updated: Friday, 13 November 2020 16:27
- Published: Thursday, 17 December 2015 16:37
- Written by Loo Kang Wee
- Hits: 8909
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The Geiger–Marsden experiment(s) (also called the Rutherford gold foil experiment) were a landmark series of experiments by which scientists discovered that every atom contains a nucleus where its positive charge and most of its mass are concentrated. They deduced this by measuring how an alpha particle beam is scattered when it strikes a thin metal foil. The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherfordat the Physical Laboratories of the University of Manchester.
The popular theory of atomic structure at the time of Rutherford's experiment was the "plum pudding model". This model was devised by Lord Kelvin and further developed by J. J. Thomson. Thomson was the scientist who discovered the electron, and that it was a component of every atom. Thomson believed the atom was a sphere of positive charge throughout which the electrons were distributed, a bit like plums in a Christmas pudding. The existence of protons and neutrons was unknown at this time. They knew atoms were very tiny (Rutherford assumed they were in the order of 10−8 m in radius). This model was based entirely on classical (Newtonian) physics; the current accepted model uses quantum mechanics.
Thomson's model was not universally accepted even before Rutherford's experiments. Thomson himself was never able to develop a complete and stable model of his concept. A Japanese scientist named Hantaro Nagaoka rejected Thomson's model on the grounds that opposing charges cannot penetrate each other. He proposed instead that electrons orbit the positive charge like the rings around Saturn.
Rutherford thus rejected Thomson's model of the atom, and instead proposed a model where the atom consisted of mostly empty space, with all its positive charge concentrated in its center in a very tiny volume, surrounded by a cloud of electrons.
The Rutherford model was devised by the New Zealand-born physicist Ernest Rutherford to describe an atom. Rutherford directed the Geiger–Marsden experiment in 1909, which suggested, upon Rutherford's 1911 analysis, that J. J. Thomson's plum pudding model of the atom was incorrect. Rutherfords new model for the atom, based on the experimental results, contained new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and with this central volume also containing the bulk of the atomic mass of the atom. This region would be known as the "nucleus" of the atom.
Computer Model Initial Values
mass of helium atom= 4u where u is the unified atomic mass number
mass of gold atom = 197u
charge of helium atom = 2e where e is the electron charge
charge of gold atom = 79e
Force is calcuated by F = kqQ/r^2 the electrostatic force equation
k = 9.0*0.03 where 0.03 is a factor scaling to scale down/up to fit the x and y dimensions of the computer model, in the nature world k = 9x10^9
q = charge of first particle
Q = charge of second particle
r is the distance that separates the 2 charged particles
the ordinary differential equations are
dx[i]/dt = vx[i]
dvx[i]/dt = xForce[i]/m[i]
dy[i]/dt = vy[i]
dvy[i]/dt = yForce[i]/m[i]
where x is the horizontal position of charge particle
i is the array index of all charge particle
t is time
vx is velocity in x direction
xForce is the vector sum of all x-direction forces calculated based on i with all the other  to [n] excluding [i] charges particles where n is 80 since it is assume there will be a maximum number of charges of 80.
m is the mass of charged particles
vy is velocity in y direction
yForce is the vector sum of all y-direction forces calculated based on i with all the other  to [n] excluding [i] charges particles where n is 80 since it is assume there will be a maximum number of charges of 80.
Auto and Manual Selection of combo-box
auto - automatically randomised the y position of a helium He particle back to the edge of the left screen and shoots in vx as the same initial vx
manual - user can drag on the helium particle to try out their choice of particle y -position and investigate the new trajectory
Rutherford Atom-Model Assumption
The gold atom cannot be moved by fixing the velocity in both x and y idrection as zero all the time.
There is a single particle at the centre of the atom, charge value = 79 u
Rutherford Atom-Model 2 Assumption
The gold atom is made up of 9 individual charge particles that cannot be moved, they are all fixed in their velocities in both x and y idrection to be zero all the time.
There is a group of 9 particles spread out tightly at the centre of the atom, each particle charge is 79u/9. This is a mathematical division as it was too tedious to have 79 particles so to test the model, I just created 9 particles to approximate instead
Thomson Atom-Model configurable Assumption
The gold atom is made up of 9 spread out individual charge particles that can be moved or reposition, they are all fixed in their velocities in both x and y idrection to be zero all the time.
There is a group of 9 particles spread out loosely at the atom, each particle charge is 79u/9. This is a mathematical division as it was too tedious to have 79 particles so to test the model, I just created 9 particles to approximate instead
Thomson Atom-Model configurable each atom Assumption
The gold atom is made up of 79 spread out individual charge particles that can be moved or reposition for exploration and learning, they are all fixed in their velocities in both x and y idrection to be zero all the time.
There is a group of 79 particles spread out loosely at the atom, each particle charge is 79u/79 = 1u. This is the best computer model for Thomsons model of the atom
Experimental basis for Rutherford model
Through conducting inquiry activities on these 4-option computer model, students can obtain computational data (visualise via the histogram or otherwise the He particle trails) and behave like scientists and make conclusion based on evidence rather than from reading texts and looking at static pictures.
Rutherford overturned Thomsons model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny and heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. If Thomson was correct, the beam would go straight through the gold foil. Most of the beams went through the foil, but a few were deflected.
Rutherford presented his own physical model for subatomic structure, as an interpretation for the unexpected experimental results. In it, the atom is made up of a central charge (this is the modern atomic nucleus, though Rutherford did not use the term "nucleus" in his paper) surrounded by a cloud of (presumably) orbiting electrons. In this May 1911 paper, Rutherford only committed himself to a small central region of very high positive or negative charge in the atom.
Rutherford's Experiment: Nuclear Atom by uploaded by HerrPingui
Rutherford experiment animation by owigger
Rutherford Gold Foil Experiment - Backstage Science
Rutherford Scattering by xmphysics
- Understanding Rutherford's gold foil experiment by Concord Consortium. Rutherford shot alpha particles at atoms to see if the positive part of atoms would interact strongly with the positve alpha particles. Explore how concentrating positive charge affects the electirc field generated by that charge and how that field affects the path of the positive alpha particles.
- JJ Thomson wrong model http://www.kcvs.ca/site/
- Rutherford experiment result and new model http://www.kcvs.ca/site/
- https://phet.colorado.edu/en/simulation/rutherford-scattering by PhET