Question

why it is believed that atom has most empty spaces

Answer 1

It is not that Atoms are ONLY filled with empty spaces in them.Atoms are filled with electrons. It's true that a large percentage of the atom's mass is concentrated in its tiny nucleus, but that does not imply that the rest of the atom is empty.
Rather, it implies that the rest of the atom has relatively low density...

Hope this helps you out!!

Answer 2

Actually it was known through an experiment performed by Rutherford and his students. This is famous by the name “Rutherford's nuclear model of Atom.” In Rutherford's famous a-particle scattering experiment it was observed that most of the a-particles passed through the gold foil undeflected.

From this he drew the conclusion that most of the space in an atom is empty as most of the alpha-particles passed through the foil undeflected.

Hope it helps you✌✌✌✌.....