Eli5 why is that an atom is 99,6 % empty space ?

Question

Eli5 why is that an atom is 99,6 % empty space ?

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Answer 1

Science is mostly apt at answering “how” questions, rather than “why” ones.

Science *observes* and *describes*, finding predictive relations between measurable quantities.

Stuff is the way it is, because it is. Then we give it an explanation that is accurate and useful.

Returning on topic: early atomic physics experiments showed this phenomenon of mass distribution, that can be explained as such:

Electro-magnetic interaction, makes it so electrons have to orbitate at a certain distance from the nucleus, while strong nuclear force makes the nucleus very tightly packed. Electrons are extremely light, whereas atoms are relatively heavy. This implies that most of the mass is in the nucleus.

More sophisticated experiments show a more complex behaviour of matter at very small scales and the simplified description of an atom as a collection of tiny balls becomes inaccurate, even though still effective.

Quantum physics describes particels as “probability waves” and thus you can’t think of an electron (or nucleus) as a tiny ball, but as something “smeared” across a region of space. And you can’t think of empty space as it being really empty anymore.

Also mass at the atomic level is a combination of the “intrinsic” mass of each sub-atomic part and the “additional” mass provided by the strength of their bonding… the game grows ever more complex.

So long story short: our “planetary” description of the atom, implies a significant volumetric void inside an atom. More accurate descriptions drop this idea of such an amout of empty space within the atom for a more complex description.