Why can't we use a gas in a thermometer?
The properties of gases are such that their use in a thermometer is not possible. One of the key properties of a gas is that it expands or contracts to fill its container. If we were to put a gas inside of a thermometer, it would fill the container and any markings on the tube would be rendered useless. If we were able to measure the pressure of a fixed size sample of gas with a constant volume, we could relate the pressure and temperature to one another as they are directly proportional.
Liquids are used in thermometers because they expand and contract with temperature changes and they take on the shape of a container without filling it. Solids do not work in thermometers because they do not take on the shape of the container.