Radon, with the chemical symbol Rn and atomic number 86, is a naturally occurring element that is both a noble gas and radioactive. It is colorless, odorless, and one of the densest known gases. Its physical state is directly tied to its temperature, meaning its boiling point determines the conditions under which it exists as a vapor. Understanding this temperature is key to comprehending its behavior in the environment and its significance for public health.
Defining the Boiling Point of Radon
The boiling point of a substance is the precise temperature at which its liquid state begins to convert into a gas, assuming standard atmospheric pressure. For radon, this temperature is exceptionally low, confirming its identity as a gas under almost all normal conditions. The boiling point for radon is measured at approximately 211.5 Kelvin (K). This value translates to -61.7 degrees Celsius (°C) or about -79.1 degrees Fahrenheit (°F).
This specific temperature marks the point of equilibrium where radon exists simultaneously as a liquid and a gas. Below this temperature, cooling the gas will cause it to condense, transitioning into its liquid form.
Radon’s Phase Transition at Low Temperatures
The boiling point of -61.7 °C means that radon is always a gas in the terrestrial environment, where temperatures rarely drop close to that level. Since the boiling point is a cryogenic temperature, scientists must use specialized cooling equipment to force radon into its liquid state. This process of liquefaction is necessary to isolate and study pure radon samples in a laboratory setting.
Radon’s freezing point, the temperature where the liquid turns into a solid, is only slightly lower than its boiling point. The element solidifies at approximately 202 K, which is equivalent to -71 °C or -96 °F. When solid radon is cooled even further, it exhibits phosphorescence, glowing a brilliant yellow that shifts to an orange-red color.
Why Radon’s Gaseous State Matters for Public Health
The fact that radon is a gas at room temperature is the single most important factor determining its impact on public health. Radon gas originates deep within the earth as a decay product in the natural radioactive chain of uranium and radium. Because it is a gas, it can easily migrate and seep out of the soil, through cracks in foundation slabs, and into buildings.
Once inside an enclosed space like a basement or home, the gas can accumulate to elevated concentrations, especially in areas with poor ventilation. Radon itself is chemically unreactive and is mostly exhaled after inhalation. The primary danger comes from its short-lived decay products, which are solid radioactive particles like polonium, bismuth, and lead.
When inhaled, these solid decay products can attach to lung tissue, where they continue to decay and emit damaging alpha radiation. This radiation can damage the cells and genetic material in the lungs, increasing the risk of developing lung cancer. Radon exposure is considered the second leading cause of lung cancer overall, after smoking. The inherent gaseous nature of radon facilitates its entry and accumulation, transforming it from a harmless underground element into a significant indoor air contaminant.