When positively charged particles were radiated onto a gold atom, a few of the particles bounced back. Which of the following caused this?
no
The phenomenon described in the question is known as the Rutherford scattering. It was an experiment conducted by physicist Ernest Rutherford in 1911, where he bombarded a thin gold foil with positively charged particles (alpha particles). Based on the observations, Rutherford concluded that the positive charge and most of the mass of an atom resides in a small, dense region called the nucleus.
Now coming back to the question, when positively charged particles were radiated onto a gold atom, only a few of them bounced back. This can be attributed to the fact that the gold atom's nucleus contains a highly concentrated positive charge, while the alpha particles also have positive charges. So, when an alpha particle approaches the gold atom, there is a repulsive force between the positively charged alpha particle and the gold atom's nucleus.
According to Rutherford's model, since the nucleus is very small compared to the size of the atom, most of the alpha particles would pass through the gold foil without significant deflection. However, a few alpha particles, due to their close approach to the comparatively massive and positively charged nucleus, would experience a strong repulsive force and bounce back in the opposite direction. These rare collisions with the nucleus caused the observed scattering of the particles.
Therefore, what caused the bouncing back of the positively charged particles in this experiment was the repulsive force between the positively charged alpha particles and the gold atom's positively charged nucleus.