How do we know that most of an atom is empty space? And since that is so, why can’t we just walk through solid objects?
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We can shoot neutrons or other weak or not charged particles through an atom to measure its size. It will only bounce off if it hits the core.
We can not walk through solids because we are not made out of uncharged particles, the electrons of your bodies atoms repell the electrons of the other object.
what you call “touching” (or “bumping against a solid object, hurting yourself”) is actually the electric field of the molecules in your body interacting with the electric field of the object you’re close to.
think of it like a bunch of magnets, except that the ramp up much faster and at a much shorter distance.
Think of it like a net. A net is mostly empty space it we can’t walk through it. There are bonds on energy holding all the atoms together. It takes some amount of energy to break those bonds like it does to break the net.
The vast majority of a large cardboard box is empty space. You can’t just walk through one though.
Describing an atom as “mostly empty space” is a simplification that isn’t quite right – because talking about “empty” versus “filled” space at atomic and subatomic scales doesn’t really make sense.
The image of electrons, protons and neutrons as solid ‘balls’ made of something solid with a clear boundary is convenient for many applications, and makes for a good intuitive image – but it’s also wrong in some regards.
Atom-atom and particle-particle interactions don’t happen because particles are “solid” and bump into another – they happen because of *forces* transmitting the interaction. For example, the reason you can’t walk through a solid object is because the electrons in your body and the electrons in the solid object repel each other – to you, atoms are effectively solid balls because you interact electrically.
To a particle that doesn’t interact electrically (such as a neutron), atoms are essentially made of mostly empty space – because the neutron can only interact with the atom’s nucleus via the strong nuclear force, and the strong nuclear force has a much shorter range than the electromagnetic force.
Electron clouds are most of an atom’s volume, and are by no means empty space. Light and other atoms bounce off electron clouds, just as in classical systems light and pool balls will bounce off other pool balls. Electron clouds are *not* tiny electrons zipping around a vast volume, they are smeared across the whole volume in a somewhat unintuitive way, because they are not classical systems (a S orbital has the highest positional probability right in the center, that is, superimposed over the nucleus which is supposably repellent and dense, for instance). Electron clouds being filled space isn’t an approximation arrived at from uncertainty in the electron’s position, it’s a well-described and (in the case of a single hydrogen atom) fully solved result of the wave equation of an electron.
Electron clouds are low-density compared to nuclei (nuclei are also clouds, as it turns out, just more massive and with smaller wavelengths), so most of the mis-conceptions about atoms being “empty space” actually map accurately on to the fact that atoms are mostly low-density space. It just turns out that low-density is is sufficient to be solid, and to reflect light and other solids.
Take the concrete example of ionic solids. Ionic solids pack together like balls arranged in closest-packed arraignments. The differences in the radii of the ions (they’ll be at least one negative and one positive in each ionic solid) determine the overall density, how strongly attached the ions are, and whether there’s space in between the balls for water molecules to wiggle into. You can note that the rule concerning water molecules is based off a specific and fixed size of water molecules, and specific and fixed radii of the ions. The ions act exactly like rigid balls, because their electron clouds are producing that exact effect. Opposite charged ions can only approach so close before their rigid edges come into contact, and their spacing leaves only a fixed size of gap for other molecules to fit through. This to me adds up to atoms which are in no way mostly empty, and are in fact solid objects.
We know that an atom is empty space because of experiments, especially shooting small stuff through the atom and seeing how much makes to the other side.
We do not cross stuff for the same reason that you can’t join two magnets on the same side, the outer part of atoms is all – and will mostly repel each other and will do it increasingly strongly the closer it gets.
Other answers have explained the second question well, but to the first, we know the atom is mostly empty space thanks to Ernest Rutherford’s experiments in the early 1900s (https://en.m.wikipedia.org/wiki/Geiger%E2%80%93Marsden_experiments). He had a sheet of gold foil 1 atom thick, and would fire alpha particles at it. Almost all of the particles went through, but some didn’t and would deflect in various directions. The particles that made it through obviously hit nothing, but the ones that deflected hit something, specifically the nucleus. These sets of experiments proved that the nucleus was at the center of the atom, and the electrons scattered widely around that. This experiment disproved the existing theory, also known as the “plum pudding model”.
MAGNETS MY BOY! MAGNETS ARE THE KEY!
Jokes aside, it’s because of electromagnetism; Opposite magnets attract and same ones repel
In this case, say you sit on a chair; The electrons of your butt are repelling the electrons of the chair (electrons are negatively charged)
If you could somehow push hard enough to get past those electrons, that’s what a nuclear fusion reactor does: Butt + Chair = KaBOOM.
this question reminded me of one of my all time favorite movies, Mindwalk (1990).
They cover the empty space of atoms in a [very ELI5 way.](https://www.youtube.com/watch?v=FvZ7RPuiBYY&list=PL1A5A55D5BAEE3C17&index=6)
“Matter is solid because probability patterns are difficult to condense?”
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