In the Gold foil experiment, most of the alpha particles bounces back because in an atom , most of the space occupied by the  nucleus, which is positively charged . So it bounces back the positively charged alpha particles.

Rutherford's Experiment and Observations

Rutherford conducted an experiment known as the gold foil experiment to investigate the structure of the atom. In this experiment, he directed a beam of alpha particles at a thin gold foil and observed their behavior. The observations made by Rutherford in his experiment were as follows:

a) Very few alpha particles deflected at small angles:
- Rutherford expected the alpha particles to pass straight through the gold foil with minimal deflection.
- However, he observed that a small number of alpha particles were deflected at small angles, which was unexpected.

c) Only a few alpha particles were scattered at large angles:
- Rutherford also observed that only a few alpha particles were scattered at large angles.
- This observation indicated that the positive charge and mass of the atom were concentrated in a small region, which he called the nucleus.

d) Most alpha particles passed straight through the gold foil:
- The majority of alpha particles passed straight through the gold foil without any deflection.
- This observation supported the idea that atoms are mostly empty space, with the positive charge concentrated in a small, dense nucleus.

Explanation of the Correct Answer

The correct answer is option 'B' - a large number of alpha particles bounced back completely. This observation was not made by Rutherford in his experiment. Instead, he observed that only a small number of alpha particles were deflected at small angles and scattered at large angles. The majority of alpha particles passed straight through the gold foil without any significant deflection.

The reason why a large number of alpha particles did not bounce back completely is due to the fact that most of the atom's mass and positive charge is concentrated in a small region called the nucleus. The nucleus is tiny compared to the overall size of the atom, and the alpha particles are much larger in comparison. As a result, most of the alpha particles passed through the empty space of the atom and interacted very little with the positively charged nucleus.

However, a small number of alpha particles did experience a significant deflection or scattering. This indicated that there must be a concentrated positive charge within the atom, which repelled the positively charged alpha particles. Rutherford's observations led to the development of the nuclear model of the atom, where the positive charge and mass are concentrated in the nucleus, while the electrons orbit around it in empty space.