What actually is the empty space in an atom?

Factual Questions
1

What actually is the space between the nucleus and an electron in an atom?

2

Complicated. The problem is that the nucleus and (especially) the electron don’t act like teeny tiny billiard balls, but like a combination of particle and wave properties that can be described mathematically but not really understood intuitively. The simplest description is that the space is (mostly) not really empty but filled with some greater or lesser degree of “electron wave”.

3

nm

4

As I understand it, the empty space is where an electron is least likely to be at any given time.

5

Ah, but what is the empty space? If the electron isn’t at that point, is there still an electron wave?

6

You can’t say that the electron isn’t there until you’ve collapsed its waveform.

I’m not sure if you could collapse all the waves in the vicinity of a hydrogen nucleus. There would be photon waves and gravity waves all the time, right?

7

Fields. The whole universe is packed to the rafters with fields.

8

The only good answer is “probably the electron”

Atoms don’t really look like miniature solar systems. There is, for the lowest state, a spherically symmetric probability density “cloud” that describes the likelihood of finding the electron there.

The atoms looks like this:

Not like this:

So the atom isn’t “mostly empty space”. It’s mostly the chance of finding an electron there.
States above the ground state have different probability distributions, which REALLY challenge your intuition of the Ways Things Ought To Behave.