The History Of The Atom Part 4

Ernest Rutherford and James Chadwick

By Alfie Mussett 13Y

Ernest Rutherford (30 August 1871 – 19 October 1937)

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Ernest Rutherford was born in 1871 in Nelson, New Zealand where he was the fourth child of James and Martha Rutherford.

His research often involved the use and study of alpha particles since he classified them in 1898. Sometime around 1909 Rutherford began to notice that alpha particles did not always act as the “plum pudding” model of the atom said they should, when they were fired at a piece of gold foil. Rutherford devised an experiment that would hopefully lead him to understand this behaviour and this experiment has since been known as Rutherford’s Gold Foil Experiment.

Rutherford’s Gold Foil Experiment
As part of his experiment, Rutherford had two associates, Hans Geiger and Ernest Marsden) aim a beam of alpha particles at a very thin gold foil, about 8.6xE-6 centimetres thick. Rutherford actually included several kinds of foils, like aluminium, iron and lead but gold foil is the one most commonly spoken of. Only Geiger and Marsden actually did the experiment, Rutherford just used the results produce a conclusion.
Here’s a diagram of the experiment carried out:

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According to the “plum pudding” model of the atom, the alpha particles should pass through the gold foil without a problem, since the “plum pudding” model said that the atom was empty space that any particle could pass through. Most of the particles followed this statement, but a few particles deviated from the expected course and most shockingly, 1 in every 20,000 particles would deflect 90 degrees from the original beam. Rutherford famously described this phenomenon with this quote: “It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you.”
The conclusion Rutherford reached was that the deflections were a result of particles running into massive concentrations of positive charge and since like charges repel, the particles were repelled by it. Thomson’s “plum pudding” model of the atom, said that positive charge was spread thinly over the whole atom, did not come close to explaining the results.
Therefore, in 1911, Rutherford came up with a new model of the atom. This new model posited that all of the positive charge in an atom is crammed into a tiny nucleus that is ten thousand times smaller than the atom as a whole. He assumed the much lighter electrons lay outside the nucleus. A key point of this theory meant that atoms, the thing that every complex organism and structure comprises of, are mostly empty space. Rutherford’s model helped make significant progress in understanding the world on the atomic scale but there was a problem with the model, the nucleus and its surrounding electrons are oppositely charged and therefore attracted one another and there didn’t appear to be anything to prevent the electrons from immediately being pulled into the nucleus. Taken at face value, this model posits that all atomic matter ought to have already imploded. Rutherford countered this by saying the atom was roughly analogous to a mini solar system, the electrons orbiting the nucleus in a similar fashion to how planets orbit the Sun. This model of the atom is the one commonly taught in schools and it’s an appealing and relatively straightforward explanation of the atom, but it was pretty well established that the model was incomplete almost as soon as it was created
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James Chadwick (20 October 1891 – July 24 1974)

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James Chadwick was born on the 20th October 1891 in Manchester and he was the eldest son of Anne Mary Knowles Chadwick and John Joseph. Chadwick was admitted to Victoria University where he was more interested in studying mathematics but he ended up studying physics because he mistakenly ended up in the wrong registration line when choosing his course. Chadwick was a bashful individual so he didn’t make any attempt to fix the mistake. He graduated in 1911 and continued his studies in the lab of Ernest Rutherford.
In 1913, Chadwick received a scholarship to study in Germany, placing him there at the same time World War I began. As a result, Chadwick was detained as a prisoner of war for four years. He returned to England in 1919 to carry out research at Cambridge University and during this time Rutherford had come up with a new model of the atom that theorized the existence of a non-charged particle in the nucleus. But Chadwick was discovering that this non-charged particle wasn’t the only particle in the nucleus. As he studied atomic disintegration, he found that the atomic number was less than the atomic mass. Since electrons have almost no mass, it would seem as though something besides the protons in the nucleus were adding to the mass. One explanation was that there were electrons and additional protons in the nucleus as well, the protons still contributed their mass but their positive charge was cancelled out by the electrons negative charge. Rutherford put the idea out that there was a particle with mass but no charge; he imagined it as a paired proton and electron. Neither idea had any evidence at the time.

Chadwick kept the problem in mind while working on other things. He then took a method for tracking particle radiation from experiments performed in Europe with the goal of looking for a neutral particle, a particle with the same mass as the proton but with no charge.

In his experiment, alpha particles were projected towards beryllium target. The emitted particles were allowed to fall on paraffin wax, which in turn released another type of particle. The study of the properties of such particles showed that they were protons. From the energy calculations, Chadwick showed that the particles released from beryllium, as a result of the incidence of alpha particles on it, were uncharged and had the same mass as protons. He called them neutrons.

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The particles (neutrons), resulting from the bombardment of beryllium with alpha particles, on falling on paraffin wax, interacted elastically with hydrogen atoms. As a result of this, neutrons stopped and protons (hydrogen nuclei) were ejected from paraffin.

There is a clear difference between protons and neutrons. Neutrons are uncharged for that they have high penetration power on traveling through a certain medium. This is not the case of protons.

Cavendish’s experiments were successful. He was able to determine that the neutron did exist and that its mass was about 0.1 percent more than the proton’s. He published his findings with characteristic modesty in a first paper entitled “Possible Existence of Neutron.”
This finding rapidly gained traction in the scientific community and Werner Heisenberg then went on to prove that the neutron could not be the “proton-electron” pairing that Rutherford posited, it had to be its own unique particle.
Chadwick received the Nobel Prize in Physics for discovering the neutron in 1935.

Sources:
1.) Name: Rutherford’s experiment and atomic model
URL:
http://www.daviddarling.info/encyclopedia/R/Rutherfords_experiment_and_atomic_model.html
Type: Website
Author: David Darling
2.) Name: Nobel Lectures, Chemistry 1901-1921

URL:http://www.nobelprize.org/nobel_organizations/nobelfoundation/publications/lectures/index.html

Type: Book
Author: The Nobel Foundation
3.) Name: The Gold Foil Experiment
URL:
http://myweb.usf.edu/~mhight/goldfoil.html
Type: Website
Author: N/A

4.) Name: A Science Oddysey: People And Discoveries
URL:
http://www.pbs.org/wgbh/aso/databank/entries/dp32ne.html
Type: Website
5.) Name: Famous Scientists
URL:
http://www.famousscientists.org/james-chadwick/
Type: Website

6.) http://library.thinkquest.org/C001124/gather/aexp.html

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