Why did most of the alpha particles go straight through the gold foil in Rutherford's experiment?

because the atom is mostly empty space

The fact that most alpha particles went straight through the foil is because the atom is mostly empty space.

Those that passed straight through did so because they didn't encounter any nuclei.

Explanation:

Ionization energy is the energy needed to remove an electron from an atom's valence shell. It increases from left to right, across the period, and decreases from top of bottom, along the group. With this info, you can figure it out. Hope this helps. Have a nice day.