How did Ernest Rutherford's alpha particle scattering experiments help him to reject the 'plum pudding' model of the atom and create a more accurate model?
2 Answers
If the 'plum pudding model' was correct, the beam of alpha particles would go almost straight through the gold foil. The fact that it did not was evidence that Rutherford used to develop a new model.
Explanation:
Sir J J Thompson developed what he actually called a 'raisin cake model', but is often called a 'plum pudding model'. Once he had discovered the electron it was clear that the atom was made up of smaller particles.
The plum pudding model suggested that the atom was a large 'cake' of positive charge, with negatively charged electrons as 'plums' within it.
Alpha particles have a positive charge, and their mass is more than 7000 times the mass of an electron. If the plum pudding model was accurate, a beam of alpha particles would be expected to pass almost perfectly straight through a piece of gold foil. They would be slightly attracted to the negatively charged electrons, but the mass of the electrons is so small compared to the mass of the alpha particles that the deflections would be tiny.
What Rutherford observed was that
"It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
The fact that most of the alpha particles passed straight through, but some bounced back, suggested a large mass with a small size and a large positive charge in the atom. It turns out that the nucleus of a gold atom has a mass about 50 times as great as an alpha particle and a positive charge about 40 times as great.
This experiment led Rutherford to develop a model in which most of the mass of an atom is concentrated in the centre, in something we call the 'nucleus'.
Ernest Rutherford expected a major percentage of the rays to pass through the gold without deflection.
Explanation:
He thought only some of the particles would deflect back.
However, even though around 60% of the alpha particles passed through without deflection, some deflected through the gold foil in small and large angles some while some alpha particles exactly reversed their path.
He hence concluded that an atom has a lot more vacant space than the thought and that most of the mass is concentrated in the centre of the atom, or its nucleus.
Since alpha particles are also positively charged, when they hit the centre of the nucleus, they reversed their path due to positive charges present there.
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