Why is solid hard answer through kinetic particle theory?

Answer 1

According to the kinetic theory of solids (kinetic particle theory) solids, the molecules in a solid vibrate around a fixed location in the solid. The molecules are in a low enough energy state that it requires a lot of additional energy to displace a large number of them simultaneously from their normal neighborhood where they oscillate. The lower the temperature, the smaller the range of their oscillation. Molecules in liquids and gasses are higher energy with much wider range of vibration and no fixed location that they vibrate around, so displacing a bunch of molecules requires much less energy.

Because the molecules in solids are much more closely packed than in gases and strongly resist any significant displacement from their preferred range of oscillation, this manifests as "hardness". Solids that resist displacement less are softer.

Answer 2

In a solid, particles are held tightly together in a fixed position, which restricts their movement. When force is applied to the solid, the particles vibrate around their fixed positions instead of moving freely, making the solid hard. This lack of mobility and the strong intermolecular forces between particles contribute to the solid's rigidity.