Antman is capable of changing his size to become extremely small. To do this Antman doesn’t lose any atoms, but shrinks all his atoms down to a new size. If he did lose atoms his brain and body would not function properly. This means he has the same mass, but a smaller volume giving him a huge density.

 

An atom is mostly empty space (more than 99.999999 %) between the electronsand the nucleus. So you would think shrinking it wouldn’t be a problem, you just reduce the free space. In fact if we could remove the space in atoms, the whole human race would fit into a space the size of an apple.

 

The problem with shrinking atoms is that electrons are restricted to specific ‘shells’ around an atom and the atom would break down if the electrons moved closer to the nucleus. In fact the electrons would be absorbed into the protons in the nucleus which would turn into neutrons (as they do in a neutron star). This leaves you with sub-atomic particles that are no longer part of atoms.

 

The comics get around this by having ‘Pym particles’, these particles discovered by Antman somehow allow the atom to shrink without collapsing.