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How does Rutherford’s model of the atom account for the back – scattering of alpha particles directed at the gold leaf
Rutherford's model of the atom can explain the phenomenon of the back-scattering of alpha particles directed at a gold leaf. To understand this, we need to know a bit about Rutherford's model and the experimental setup used.
1. Rutherford's model of the atom:
Rutherford proposed a model of the atom in 1911, known as the "Planetary model" or "Rutherford model." According to this model, atoms have a small, dense, positively charged nucleus at the center, surrounded by negatively charged electrons orbiting around it.
2. The experimental setup:
Rutherford, along with his colleagues Hans Geiger and Ernest Marsden, conducted an experiment to validate the model. They directed a beam of alpha particles (consisting of two protons and two neutrons) at a thin sheet of gold foil. They then observed the pattern of scattering of these particles.
Now, coming to the question of how Rutherford's model accounts for the back-scattering of alpha particles:
3. Expectation based on the model:
In Rutherford's model, most of the mass and positive charge of an atom is concentrated in a tiny nucleus. Since the alpha particles are positively charged, they should experience some deflection or scattering as they pass close to the positively charged nucleus. However, Rutherford expected that most of the alpha particles would pass straight through the gold foil with only slight deflections.
4. Observation and explanation:
Contrary to expectations, Rutherford and his team observed that a small fraction of the alpha particles were extremely deflected, and some even bounced directly back ("back-scattered"). This observation challenged the prevailing understanding of the atom's structure at the time.
To explain this unexpected result, Rutherford proposed that the atom's nucleus must be extremely small and densely packed with positive charge. Only by having such a concentrated positive charge could the alpha particles experience strong repulsion and significant deflections. The back-scattering of the alpha particles indicated that they were almost hitting the nucleus head-on.
So, in summary, Rutherford's model suggests that the back-scattering of alpha particles directed at the gold leaf occurs because most of the atom's mass and positive charge is concentrated in a small, dense nucleus. The strong positive charge in the nucleus repels the positively charged alpha particles, leading to their deflection and occasional back-scattering.