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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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.
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