Which particular observation of Rutherford do you think him to believe that the atoms is mostly empty space and that its mass is concentrrated in apositve nucleus?

Answer 1

The fact that the vast majority of the alpha particles got straight through led Rutherford to propose that the atom was composed primarily of empty space.

The fact that backscattering occurred in 1 in 8000 alpha particles indicated that there was a small massive positively charged nucleus in the centre of the atom.