Question

who discovered that the atom is mostly empty space

Answer 1

Rutherford discovered that the atom is made of mostly empty space.

Explanation:

He knew that an atom was made of mostly empty space because most particles passed straight through the foil.

Answer 2

Rutherford carried out a fairly simple calculation to find the size of the nucleus, and found it to be only about 1/100,000 the size of the atom. The atom was mostly empty space.