After rutherford bombarded gold foil with alpha particles, he concluded that the volume of an atom is mostly empty space. which observation led rutherford to this conclusion?
1. some of the alpha particles were deflected 180 degrees
2. the paths of deflected alpha particles were hyperbolic.
3. many alpha particles were absorbed by gold nuclei.
4. most of the alpha particles were not deflected.

4. Most of the alpha particles were not deflected.

Explanation:

Rutherford gave an experiment known as gold foil experiment.

In his experiment, he took a gold foil and bombarded it with alpha particles (carrying positive charge). He thought that the particles will pass straight through the foil, but to his surprise, many of them passed through, some of them deflected their path and a few of them bounced back.

Due to the presence of positive charge on the alpha particles they must be deflected, but as there exists a large empty space as most of the alpha particles pass through without deflection.  

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.