Why does thermal conduction occur more slowly in gases than in solids?

Thermal conduction occurs more slowly in gases than in solids primarily due to the differences in particle density and the frequency of molecular collisions.1

In a solid, atoms and molecules are packed closely together, allowing thermal energy to be transferred efficiently through atomic agitation and direct contact between particles. Because the particles are in constant proximity, energy moves rapidly along a temperature gradient until the material reaches thermal equilibrium.1

In contrast, when matter is in the gas phase, particles are separated by much larger distances. Consequently, these particles collide with one another far less frequently than they do in a solid. Since these collisions are the primary mechanism for transferring thermal energy in a gas, the reduced frequency of interaction results in a significantly lower rate of thermal conductivity. While gases do transport energy through both particle agitation and the physical movement of molecules, the sparse nature of the gas phase makes this process inherently less efficient than conduction in solids.1