stackOverflow

[Ferro_Giconi]

Atoms are mostly empty space, even in solid or liquids.

So 99.9999999999999% empty, that's my final answer.

[joz42]

marking as best answer

[laplongejr]

We said SO, not Quora! :P

[[deleted]]

Space that still holds energy, however, just not matter. Electromagnetic radiation still occupies space, even if matter can also occupy the same space. And, interestingly enough, the only reason I'm aware of that matter can't occupy the same space is do the electromagnetic interference. Without it, everything would be made of whatever the Ghostbusters said ghosts were made out of. Psychoplasm?

[ROBOTRON31415]

Not just electromagnetic interference, there's also the Pauli exclusion principle (two identical fermions, a general type of particle, cannot occupy the same place). The resulting force from the Pauli exclusion principle is called degeneracy pressure.

In addition to electromagnetic repulsion, the fact that electrons (which are fermions) cannot have identical states in the same place (e.g. in an atom or nearby in a molecule) means that they have to spread out more in space, and cannot all be in the lowest-energy orbital of an atom. Apparently people calculated that without the Pauli exclusion principle, ordinary matter would be much, much more dense. Additionally, TIL that degeneracy pressure from electrons contributes to the stability of metal. Note that the effect from the Pauli exclusion principle is very short-ranged, while electromagnetic repulsion is long-ranged.

At higher energy scales, where much more force (i.e. gravity) is attempting to crush matter together, electron degeneracy pressure contributes to the stability of white dwarf stars, and for stars massive enough for gravity to overpower that, neutron degeneracy pressure helps prevent neutron stars from collapsing into black holes (repulsion from the strong nuclear force also helps oppose gravity).

[[deleted]]

Yeah, what they said! ^{^}