Which gas has the lowest thermal conductivity?

Hint: In order to solve this question, we should first understand the concept of thermal conductivity. The property of a material to conduct heat is usually known as thermal conductivity, here we wil discus the thermal conductivity of gases.

Complete answer:
The thermal conductivity of any material is a measure of the ability of that certain material to transfer or conduct heat. Conduction generally occurs when a temperature gradient is present across the given material. It is usually denoted either by \[\lambda \] or $k$. The transfer of heat occurs at a lower rate in materials which have low thermal conductivity.

For example, the metals which have high thermal conductivity and which are very good at conducting heat. The heat transfer rate of an ideal gas is proportional to the average molecular velocity, molar heat capacity, the mean free path of the gas.

The thermal conductivity usually increases when the average particle velocity is increased. This is because of the following reason: The increase in average particle velocity increases the forward transport of energy of an ideal gas. The gases having a high density such as xenon and dichlorodifluoromethane have the least thermal conductivity.

Therefore, xenon and dichlorodifluoromethane have the lowest thermal conductivity.

Note: Among the three states of matter, solids, liquids and gases, gases usually have lower relative thermal conductivities because their molecules are not tightly packed like those in solids. Thus, heat transfer is more dependent on the free movement of the molecules and the velocity of molecules.