Rutherford’s Alpha-Particle Scattering Experiment
Rutherford's Alpha-Particle Scattering Experiment
Early Views of the Atom
i. Around 400 BC a Greek scientist called Democritus said that matter was made up of small particles he named 'Atoma' (meaning indivisible).
ii. In 1804 John Dalton stated that matter consisted of tiny solid balls he called 'Atoms'.
Backdrop of Rutherford's Experiment
At the turn of the century, there was little known about atoms except that they contained electrons. J. J. Thompson discovered the electron in 1897, and there was considerable speculation about where these negatively charged particles existed in nature. Matter is electrically neutral; some positive charge must balance the charge of the electron. These was what the scientist thought at that time.
One popular theory of the time was called the 'plum-pudding model'. This model, invented by Thompson, envisioned matter made of atoms that were spheres of positive charge spiked with electrons throughout. Electrons were chunks of plum distributed through a positively charged sphere of pudding.
The Experiment
The 'plum-pudding model' was accepted until the famous experiment - Rutherford's alpha particle scattering experiment' was carried out. Actually, the two students of Rutherford -Geiger and Marsden were asked to carry it out.
They fired a sheet of gold foil by the alpha particles. The alpha particles were emitted from a sample of Uranium. They were not absorbed before being detected because the foil is naturally electrically neutral.
Rutherford expected all the alpha particles to go through the foil, as he believed Thompson's 'plum-pudding' atomic structure.
But, the results of the alpha ...
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The Experiment
The 'plum-pudding model' was accepted until the famous experiment - Rutherford's alpha particle scattering experiment' was carried out. Actually, the two students of Rutherford -Geiger and Marsden were asked to carry it out.
They fired a sheet of gold foil by the alpha particles. The alpha particles were emitted from a sample of Uranium. They were not absorbed before being detected because the foil is naturally electrically neutral.
Rutherford expected all the alpha particles to go through the foil, as he believed Thompson's 'plum-pudding' atomic structure.
But, the results of the alpha particle scattering experiment were surprising. What they observed was not really identical as their expectation. The record on the screen showed that when the alpha particle traveled through the gold foil, some of the particles were bent in different angles but not in a straight line.
They observed the path of the alpha particles, which traveled through the foil:
. The vast majority of the alpha particles are deflected very little as they travel through the foil;
2. A tiny minority are deflected through large angles or rebounded.
Conclusions
Through these results, they drew some imagination:
. The nucleus is so small that the odds are overwhelmingly in favor of a given alpha particle motoring right on through the gold foil as if nothing were there. It turns out that the atom is a very empty place.
2. Some alphas, by pure random chance, will pass near some gold atom nuclei during their passage through the foil and will be slightly deflected. By pure chance, some or all of the small deflections will add up and shove the alpha particle off a straight-line path. Those alphas will emerge slightly deviated (say one or two degrees) from a straight-line path.
3. A very, very few alphas, by pure, random chance, will hit a nucleus almost head-on. The alpha, traveling at 10% the speed of light, penetrates the atom and gets very close to the nucleus. However, the repulsion between the alpha and the atom nucleus is so great that the atom flings the alpha back out, and it does so in a hyperbolic path. Depending on various factors, this occasionally results in the alpha being turned around 90° or more. The very heavy nucleus recoils a bit from the impact, but essentially goes nowhere.
Through these accounts, they drew two important conclusions:
. An atom's mass must be concentrated in a small positively charged nucleus.
2. Most of the atom must be empty space. This space must contain the electrons.
Atomic and Nuclear Sizes
On replacing the gold foil by aluminum foil (some years later), it turned out that small angle scattering obeyed the above law, but large angle scattering didn't. Rutherford correctly deduced that in the large angle scattering, which corresponded to closer approach to the nucleus, the alpha was actually hitting the nucleus. This meant that the size of the nucleus could be worked out by finding the maximum angle for which the inverse square scattering formula worked, and finding how close to the center of the nucleus such an alpha came.
The diameter of a gold atom is about 0.3×10-9m (=0.3nm). The nucleons are made up of quarks, and have a radius of about 0.8 fm (0.8 ? 10-15) in diameter. If an atom were the size of a football stadium, with the electrons out around the upper deck, the nucleus down at midfield would be smaller than the coin flipped at the start of the game.
If the nucleus were represented in a model by a pea (which had a length of 5-7mm):
Let the diameter of a pea is 6mm.
The diameter of the atomic model is 6 × 105 mm = 600 m
The volume of the atomic model is 3003 × 3/4 × ? ˜ 400m × 400m × 400m
Therefore, the atomic model should be a sphere which has a radius of 300m, or a cube which has each side of 400m.
