How was it shown that an atom has a lot of empty space within it ?

How was it shown that atom has empty space ?

a. Is there any well defined boundary for a nucleus ? b. What is the order of the size of a nucleus? c. How will you conclude that an atom has a lot of empty space ?

Particles in a solid have a lot of space between them.

The empty space within hcp arrangement is

The physical state of substance with lot of empty space(intermolecular) is

Empty space in BCC lattice is -