Radium is one of the most reactive metals on the periodic table. As an alkaline earth element sitting below barium, it reacts vigorously with air, water, halogens, and acids, forming compounds that range from highly soluble salts to nearly insoluble precipitates. Its reactivity extends even into biology, where it mimics calcium inside the human body.
Reaction With Air
Freshly cut radium has a brilliant white metallic luster, but that appearance doesn’t last. The metal rapidly turns black when exposed to air, likely forming a nitride layer on its surface as it combines with nitrogen. It also reacts with oxygen, producing radium oxide. This tarnishing happens faster than it does with barium, the element directly above it in the periodic table, reflecting radium’s position as the most reactive member of its group.
Reaction With Water
Radium reacts readily with water, producing radium hydroxide and hydrogen gas. The reaction follows the same pattern as other alkaline earth metals but proceeds more quickly than barium’s reaction with water. When a piece of radium contacts water, it releases hydrogen gas (which bubbles off) and dissolves into a strongly basic hydroxide solution. The energy released is substantial, with a calculated heat of reaction around 9.0 kilocalories per gram atom. The resulting radium hydroxide is soluble in water, making radium a highly electropositive metal that gives up its electrons easily.
Reaction With Halogens
Radium combines with halogens (fluorine, chlorine, bromine, and iodine) to form halide salts. Radium dichloride and radium difluoride are the most studied of these compounds. Radium chloride and radium bromide both dissolve in water, though they are less soluble than the corresponding barium salts. An interesting quirk: while radium’s chloride and bromide are harder to dissolve than barium’s versions, radium nitrate is actually more soluble than barium nitrate. The solubilities of all these salts decrease when you add their respective mineral acids to the solution.
Reaction With Sulfuric Acid and Sulfates
When radium encounters sulfate ions, it forms radium sulfate, one of the most insoluble radium compounds known. Radium sulfate has a solubility product of 3.7 × 10⁻¹¹, making it even less soluble than barium sulfate (1.1 × 10⁻¹⁰). This extreme insolubility is important in environmental chemistry and in separating radium from other elements. If you mix a radium solution with sulfuric acid or any soluble sulfate, the radium sulfate crashes out of solution almost completely.
Reaction With Carbonate Minerals
Radium interacts with a wide range of carbonate minerals, and it does so more broadly than barium. In studies comparing the two elements, radium reacted with all eight carbonate minerals tested, including calcite, dolomite, strontianite, rhodocrosite, ankerite, witherite, siderite, and magnesite. Barium, by contrast, showed measurable uptake on only three of those: dolomite, magnesite, and siderite.
The way radium bonds to these minerals varies. With calcite, dolomite, strontianite, rhodocrosite, and witherite, radium uptake increases as its concentration rises, suggesting it gets incorporated into the mineral structure through a coprecipitation process. With siderite, magnesite, and ankerite, the behavior points to simple surface sorption, where radium atoms stick to the mineral surface without becoming part of the crystal lattice. This broad reactivity with carbonates is one reason radium can be difficult to remove from groundwater, since it binds to so many common rock-forming minerals.
Reaction With Zinc Sulfide
One of radium’s most famous interactions isn’t a chemical reaction in the traditional sense. When radium sits near zinc sulfide, a phosphorescent material, the alpha particles, beta particles, and gamma rays it emits strike the zinc sulfide and cause it to glow. This radioluminescence was the basis for the glow-in-the-dark paint used on watch dials in the early 20th century. The zinc sulfide doesn’t chemically combine with radium. Instead, the radiation delivers energy to the phosphor, which re-emits it as visible light. Alpha particles alone can produce the glow, though beta and gamma rays contribute as well.
How Radium Reacts Inside the Body
Radium’s chemistry makes it dangerous in a specific way: it mimics calcium. Because radium sits in the same column of the periodic table as calcium, the body treats it like calcium and deposits it into bones. Once there, radium binds to hydroxyapatite, the mineral that gives bone its hardness. Unlike calcium, though, radium is radioactive. Its alpha particle emissions cause double-stranded breaks in DNA, which can lead to bone cancers over time.
This same property has a medical application. Radium-223, an isotope with a half-life of 11.4 days, is used to treat bone pain in certain cancers. Each radium-223 atom releases four alpha particles as it decays, delivering targeted radiation directly to bone tissue where cancer cells have spread. The calcium-mimicking behavior that makes radium dangerous in environmental exposure becomes therapeutically useful when directed at tumors.
Why Radium Is More Reactive Than Barium
Radium’s extreme reactivity comes down to its size and electron structure. It has the lowest first ionization energy among the alkaline earth metals at 5.279 electron volts, meaning it takes very little energy to strip away its outermost electrons. The larger the atom, the more loosely it holds its valence electrons, and radium is the largest alkaline earth metal. This makes it highly electropositive, eager to shed electrons and form bonds with nearly anything that can accept them. Its chemical behavior mirrors barium’s in most respects, but every reaction proceeds a bit faster and the resulting compounds tend to be slightly less soluble, with radium sulfate being the clearest example of that trend.