Atomic number 88

Radium

Ra
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Shells Orbitals 7s

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88 electrons in 7 shells: 2, 8, 18, 32, 18, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkaline earth metal
Period: 7
Group: 2
Block: s
Atomic mass: 226.02541 Da (longest-lived isotope)
Electron configuration: [Rn] 7s²
Phase at 25 °C: solid
Melting point: 700 °C
Boiling point: 1737 °C
Density: 5 g/cm³
Electronegativity (Pauling): 0.9
Discovered: 1898, Marie Sklodowska Curie and Pierre Curie

Neighbours in the table

Fr Ac

Radium’s 1898 discovery belongs to Marie Sklodowska Curie and Pierre Curie in Jefferson Lab’s account; Los Alamos places the work on pitchblende from North Bohemia. Radium is about a million times more active than uranium, by Jefferson Lab’s estimate. Freshly prepared, the pure metal is a brilliant white, Los Alamos reports, but it blackens in air, probably as a nitride forms. It turns a flame carmine red, breaks down in water and is a little more volatile than barium. The Curies’ laboratory notebooks are still too contaminated to handle safely.

The element closes group 2 in period 7. Summed up as [Rn] 7s², radium’s 88 electrons spread across seven shells as 2, 8, 18, 32, 18, 8 and 2. Ionisation energy is the energy needed to pull an electron off. It falls down a group because each new period places the outer electron one shell farther from the nucleus, as Chem1 explains. PubChem gives radium’s first ionisation energy as 5.279 eV. Group 2 elements tend to give up their two outer electrons to form 2+ ions, Chem1 notes, and radium’s oxidation state is +2.

Radium has no stable isotope. PubChem gives the mass of its longest-lived isotope, radium-226, as 226.025 Da.

Where it is found

Radium-226 belongs to the uranium series, EPA explains, and radium-228 and radium-224 to the thorium series. Radium-226 is found in nature because uranium-238 keeps producing it as it decays, IUPAC notes. Traces of radium turn up in nearly all rock, soil, water, plants and animals, according to EPA. Jefferson Lab puts it at 9 × 10⁻⁷ milligrams per kilogram of crust and 8.9 × 10⁻¹¹ milligrams per litre of seawater.

Uranium ores hold more. Los Alamos finds roughly 1 g of radium per 7 tons of pitchblende; Jefferson Lab gives about 0.14 g per ton of uranium ore. Radium first came from the rich pitchblende of Joachimsthal in Bohemia, Los Alamos records, and Colorado’s carnotite sands furnish some. Today radium is a by-product of uranium refining, sold as the chloride or bromide rather than as metal. Curie and Debierne isolated the metal in 1911 by electrolysing radium chloride over mercury, then distilling the amalgam, a mercury alloy, Los Alamos records.

Industry concentrates it further. Uranium milling leaves radium in the tailings, the waste after ore processing, and oil and gas production can build it up in pipe scale, EPA notes.

Why it matters

For decades radium made instruments readable in the dark. ATSDR, the US Agency for Toxic Substances and Disease Registry, records that until the 1960s radium paint lit watch and clock dials, aircraft instrument panels, military instruments and compasses. Los Alamos notes that the metal and its salts are luminescent. Cobalt-60, a less dangerous radioactive source, has largely taken over this work, in Jefferson Lab’s account.

Medicine used radium in two ways. As a radiation source it treated cancer, and IUPAC’s notes place radium-226 in brachytherapy, a treatment aimed at a tumour from close range. Radium also supplied radon, which was sealed in very small tubes for cancer treatment, Los Alamos notes. Outside medicine, radium mixed with beryllium gives out neutrons when its alpha particles strike the beryllium, Los Alamos explains. The curie, a unit of radioactivity, is based on radium-226.

Its isotopes also follow moving water. Radium-223, -224, -226 and -228 behave conservatively in a water mass, so only mixing and decay change how they spread, IUPAC’s notes explain. That makes them tracers for water flow, such as groundwater seeping into the sea. Because radium-226 and radium-228 have half-lives of 1,600 and 5.75 years, they can date material up to a few thousand years old. Radium-226 has dated a limestone cave in central Switzerland and corals in the Indian Ocean.

Properties

Properties of radium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	88
Atomic mass	226.02541 Da	Da	PubChem, longest-lived isotope
Melting point	700 °C	°C	Los Alamos: 700 °C
Boiling point	1737 °C	°C	Los Alamos: 1737 °C
Density	5 g/cm³	g/cm³
Electronegativity (Pauling)	0.9
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s²
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Radium is hazardous for its ionising radiation, and Haz-Map’s radionuclide summary gives bone as its critical organ and its internal toxicity as very high. IARC, the International Agency for Research on Cancer, finds that swallowed radium-226 and radium-228 cause bone sarcomas, with radium-226 also causing cancers of the sinuses and mastoid. EPA links chronic exposure at high levels to more bone, liver and breast cancer, and notes higher exposure for uranium miners and ore-processing workers. EPA refers old radium devices to state radiation control programs; the PubChem Laboratory Chemical Safety Summary for radium is the reference sheet.

Questions

What is the half-life of radium-226?

Radium-226 has a half-life of 1,600 years, in IAEA's evaluation. IAEA puts alpha emission, which turns it into radon-222, at 100 per cent of its decays. A second, tiny branch ejects a carbon-14 nucleus and leaves lead-212: 3.2 × 10⁻⁹ per cent by IAEA, 2.6 × 10⁻⁹ per cent in the IAEA Atomic Mass Data Center table, which also lists a possible double beta-minus decay.

Does radium give off radon?

Radium-226 does: its alpha decay produces radon-222, a radioactive gas. EPA ranks radon second among causes of lung cancer in the United States. Radium-228 takes another route, decaying by beta-minus emission in 100 per cent of cases by IAEA's evaluation, and its half-life is 5.75 years.

Is there radium in drinking water?

Most water carries traces, because radium turns up in nearly all rock, soil and water, according to EPA, and drinking water generally holds little of it. Groundwater usually holds more radium where the local rocks and soils are richer in it, and some regions run higher because of their geology. Radium in water from US community systems is regulated, while private wells are not, according to EPA.

Is radium dangerous to touch?

Alpha particles travel only a short way and cannot cross the skin, according to ATSDR, the US Agency for Toxic Substances and Disease Registry, whereas gamma rays from radium's decay chain can pass right through the body. Haz-Map gives radium-226 itself as an alpha emitter, and ATSDR notes alpha, beta and gamma radiation along the chain of daughters. Los Alamos warns that breathing in, injecting or being exposed to radium can cause cancer.