Why did rutherford conclude that the atom was mostly empty space?

Rutherford came to this conclusion after his Gold-Foil experiment. He found that most of the alpha particales(when released) went straight through the gold foil, of which consisted of atoms. In the middle of the atom theres a small nucleus that has mass and positive charge. Because the nucleus is so small the alpha particles were able to go straight through leading to the conclusion that atoms are mostly empty space. 

9/210=x/413210x=3717x-3717/210x=17.7 gal