Observations:

• Most alpha particles passed straight through the gold foil.
• Some alpha particles were deflected by small angles.
• A very few alpha particles (about 1 in 12,000) bounced back.

Conclusions:

• Most of the space inside an atom is empty.
• There is a positively charged center in the atom, which Rutherford called the nucleus.
• Nearly all the mass of the atom is concentrated in the nucleus.