2. What makes up the space between atoms?

The nucleus makes up a tiny proportion of the space occupied by an atom, while the electrons make up the rest. According to quantum electrodynamics, the space is filled by an electron field around the nucleus which neutralizes its charge and fills the space defining the atom size.

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.