Geiger and Marsden began with small-angle dispersion and tried various thicknesses of foils, seeking mathematical relationships between dispersion and thickness of foil or number of atoms traversed. Marsden later recalled that Rutherford said to him amidst these experiments: "See if you can get some effect of alpha-particles directly reflected from a metal surface. Rutherford was ever ready to meet the unexpected and exploit it, where favourable, but he also knew when to stop on such excursions.
Birks, , p. This was Rutherford's playful approach in action. His students and others tried out his ideas, many of which were dead-ends. Rutherford wrote:. Experiment, directed by the disciplined imagination either of an individual or, still better, of a group of individuals of varied mental outlook, is able to achieve results which far transcend the imagination alone of the greatest philosopher. Quoted in Eve, , Frontmatter. Sometime later in or , Marsden said, he reported his results to Rutherford.
Rutherford recalled this a little differently:. I remember It was almost incredible as if you fired a inch shell at a piece of tissue paper and it came back and hit you. Rutherford , , p.
Human memory is fallible. Whether Marsden or Geiger told Rutherford, the effect was the same. Rutherford said they should prepare a publication from this research, which they submitted in May Moreover, this started Rutherford thinking toward what ultimately, almost two years later, he published as a theory of the atom. What was Rutherford doing for the rest of and all of ? For one thing, his close friend Boltwood was in Manchester for the academic year working with Rutherford on radioactive decay products of radium.
He was also reviewing and speaking on earlier ideas about atomic structure. Rutherford did not have his bold idea — the nuclear atom — instantly, but he came to it gradually by considering the problem from many sides.
In the autumn of he brought Marsden back to Manchester to complete rigorous experimental testing of his ideas with Geiger. Rutherford tried to reconcile scattering results with different atomic models, especially that of J. Thomson, in which the positive electricity was considered as dispersed evenly throughout the whole sphere of the atom. At some point in the winter of —, Rutherford worked out the basic idea of an atom with a "charged center. To produce a similar effect by a magnetic field, the enormous field of absolute units would be required.
Birks, p. Rutherford concluded in his May paper that such a remarkable deviation in the path of a massive charged particle could only be achieved if most of the mass of, say, an atom of gold and most of its charge were concentrated in a very small central body.
Note: at this point in , Rutherford did not call this a "nucleus. The first public announcement of the nuclear theory by Rutherford was made at a meeting of the Manchester Literary and Philosophical Society, and he invited us young boys to go to the meeting. The older people in the laboratory did, of course Geiger and Marsden knew because they were already doing the experiments.
In fact, unless they had done some which were sufficient to be decisive, Rutherford never mentioned it publicly. And, of course, Darwin knew about it much earlier. But that must have been early in , and we went to the meeting and he told us. And he mentioned then that there was some experimental evidence which had been obtained by Geiger and Marsden. He did not, as far as I remember, say more about the results than that they were quite decisive.
If you look at some of his papers in the early days — I call McGill the early days — he was quite convinced that the alpha particles were atoms of helium, but he never said that in those words. He always said they were either atoms of helium or molecules of hydrogen or perhaps he may have said something else of that weight.
It was quite characteristic of him that he would never say a thing was so unless he had experimental evidence for it that really satisfied him. He worked out quickly and roughly that several quantitative relationships should be true if this basic theory were correct.
Second, that number should be proportional to the square of the nuclear charge. These three ideas laid out the experimental program of Geiger and Marsden for the next year. B According to the plum pudding model top all of the alpha particles should have passed through the gold foil with little or no deflection. Rutherford found that a small percentage of alpha particles were deflected at large angles, which could be explained by an atom with a very small, dense, positively-charged nucleus at its center bottom.
Surprisingly, while most of the alpha particles were indeed undeflected, a very small percentage about 1 in particles bounced off the gold foil at very large angles.
Some were even redirected back toward the source. No prior knowledge had prepared them for this discovery. Rutherford needed to come up with an entirely new model of the atom in order to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space.
In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom. The nucleus is the tiny, dense, central core of the atom and is composed of protons and neutrons. In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the nucleus at the center of the atom. Rutherford called this news the most incredible event of his life. In the now well-known experiment, alpha particles were observed to scatter backwards from a gold foil.
Ernest Rutherford was born in New Zealand, in , one of 12 children. Growing up, he often helped out on the family farm, but he was a good student, and received a scholarship to attend the University of New Zealand. After college he won a scholarship in to become a research student at Cambridge. At Cambridge, the young Rutherford worked in the Cavendish lab with J. Thomson, discoverer of the electron.
In Rutherford and chemist Frederick Soddy found that one radioactive element can decay into another. The discovery earned Rutherford the Nobel Prize in Chemistry, which irritated him somewhat because he considered himself a physicist, not a chemist.
In Rutherford returned to England, to the University of Manchester. In , he and his colleague Hans Geiger were looking for a research project for a student, Ernest Marsden. Rutherford had already been studying the scattering of alpha particles off a gold target, carefully measuring the small forward angles through which most of the particles scattered.
Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space. In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom.
He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom's interior, which he called the nucleus. The nucleus is the tiny, dense, central core of the atom and is composed of protons and neutrons. Rutherford's atomic model became known as the nuclear model.
In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the nucleus at the center of the atom.
The electrons are distributed around the nucleus and occupy most of the volume of the atom. It is worth emphasizing just how small the nucleus is compared to the rest of the atom. If we could blow up an atom to be the size of a large professional football stadium, the nucleus would be about the size of a marble. Rutherford's model proved to be an important step towards a full understanding of the atom.
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