Radium (Ra) is a highly radioactive element known for its intense energy emissions and historical applications. Positioned on the periodic table as an alkaline earth metal, it is chemically reactive and physically unstable due to its atomic structure. Understanding its physical characteristics, such as its mass density, provides insight into its nature as a heavy metal. Because of its extreme radioactivity, measuring and studying Radium’s bulk properties presents unique scientific challenges.
Understanding Radium as an Element
Radium holds the atomic number 88, placing it in Group 2 of the periodic table alongside elements like calcium and barium, known as the alkaline earth metals. Its chemical behavior strongly resembles these lighter metals, particularly barium, due to having two valence electrons available for bonding. Radium does not exist freely in nature because all of its isotopes are radioactive and decay over time.
The most stable and common isotope is Radium-226, which possesses a half-life of approximately 1,600 years. This isotope is a natural decay product found in the radioactive series that begins with Uranium-238. Radium is only found in trace amounts within uranium-bearing ores, such as pitchblende. Marie and Pierre Curie famously discovered the element in 1898, isolating it from tons of pitchblende ore to confirm its existence as a distinct, highly active substance.
The Measured Density Value
The mass density of pure, metallic Radium is measured to be approximately \(5.5 \text{ g/cm}^3\) (\(5,500 \text{ kg/m}^3\)). For a general comparison, Radium is more than twice as dense as aluminum (\(2.7 \text{ g/cm}^3\)), but significantly less dense than iron (\(7.9 \text{ g/cm}^3\)). This density reflects Radium’s high atomic mass of 226 and its closely packed crystal structure.
Accurate measurement of this value is complicated by the element’s inherent properties. Radium’s intense radioactivity causes continuous self-heating and decomposition of surrounding materials used for measurement. Furthermore, a freshly purified sample quickly reacts with nitrogen in the air, forming a surface layer of radium nitride that alters the bulk properties. These factors contribute to slight variations in published density figures, with some sources reporting values closer to \(5.0 \text{ g/cm}^3\).
Other Important Physical Properties
When Radium is first isolated in its pure metallic form, it presents a lustrous, silvery-white appearance. This metallic sheen is fleeting, however, as the surface rapidly tarnishes to a black color upon exposure to the atmosphere due to the formation of radium nitride. Radium is a solid at room temperature and exhibits a body-centered cubic (BCC) crystal structure.
Radium has a melting point of around \(700^{\circ}\text{C}\) and a boiling point of approximately \(1,737^{\circ}\text{C}\). These thermal properties are comparable to its lighter counterparts in Group 2, following expected periodic trends. The intense radioactivity of Radium causes it to emit a faint blue glow in the dark, resulting from the radiation exciting the surrounding air.
Historical Significance and Radioactivity Risks
Radium’s primary relevance stems from its radioactivity, which led to both its historical fame and severe health risks. In the early 20th century, Radium compounds were mixed with zinc sulfide to create self-luminous paint for watch and clock dials, a practice that continued until the 1960s. Radium was also employed in various quack remedies and used in early medicine to generate radon gas for cancer therapy.
The health hazards associated with Radium are profound because the body mistakenly treats it like calcium, leading to its accumulation in bone tissue. Once deposited in the skeleton, the continuous emission of alpha, beta, and gamma radiation damages bone marrow. This damage can cause bone cancer, anemia, and other serious disorders. Due to these dangers, Radium’s use has been largely discontinued and replaced by safer, less toxic radioactive sources.