Question

Why coolant constant K depends on the medium?

If most of the space inside an atom is is actually vacuum and all the electrostatic attraction and repulsion that happens from one medium to another medium like electrons flow have to happen from from one atom to another atom and most of the space inside the atom itself is empty or vacuum so what my answer to this question would be so if most of the space inside an atom is empty then culomb constant or permitivity should depend on intermolecular spaces then medium and instead of dielectric constant or absolute permittivity it should only be permittivity of vacuum

Answer 1

The electrostatic force is NOT medium dependent. The force between two unit charges at a given distance is always the same, irrespective of the surroundings, and is given by the Coulomb's law (with constant ϵ0ϵ0). When you put charges inside a medium like water/oil, the medium around gets polarized, and the new field created due to this acts on the charges inside it. That's why it looks as if there was an "apparent" change in force between charges as compared to when they were in the vacuum.

its the medium which gets polarized cancels a part of the overall field, and the force is a measure of the net field. therefore it depends on the medium.