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General Chemistry

Radium

Definition and meaning of Radium in chemistry.
Radium is a highly radioactive chemical element with the symbol Ra and atomic number 88. It is the heaviest alkaline earth metal and belongs to group 2 of the periodic table. In its pure form, this metal looks brilliant silvery white but turns black when exposed to air.
Radium element card: symbol Ra, atomic number 88, atomic mass [226], electron configuration [Rn] 7s², oxidation states +2, Alkaline earth metal, group 2, period 7, solid at room temperature.

In more detail

Students often confuse radium with radon, but radon is a gas that forms when radium decays. Radium sits under barium on the periodic table and shares similar chemical behaviors. It reacts strongly with water to make radium hydroxide and hydrogen gas.

When exposed to air, it forms a black nitride layer on its surface. The element naturally exists in tiny amounts within uranium and thorium ores, including pitchblende. Its nucleus is unstable and constantly splits apart.

This process releases alpha, beta, and gamma radiation. The released energy ionizes nearby nitrogen molecules in the air. This ionization makes the air glow with a faint blue light.

Ionization means that the radiation strips electrons away from neutral gas molecules. Radium behaves like calcium in the human body because they are in the same chemical family. If swallowed, the body mistakenly puts radium into bone tissue instead of calcium.

This concentrated radiation leads to bone cancer and other severe health issues. The most stable isotope is radium-226, which has a half-life of about 1600 years. This half-life means it takes 1600 years for half of a sample to decay.

Key facts

Field
General Chemistry
Symbol
Ra
Atomic number
88
Atomic mass
[226] u
Category
Alkaline-Earth-Metal
State at room temperature
Solid
Example
Historically, workers painted watch dials with radium paint so they would glow in the dark. The painters, known as the Radium Girls, licked their brushes to make fine tips. They absorbed radium into their bones, which caused many of them to grow very sick. This tragic event led to strict labor safety laws and ended the commercial use of radium paint. Today, scientists handle radium with thick shielding and remote tools to prevent any exposure.

Frequently asked questions

Why does radium glow in the dark?

Radium releases high-energy radiation that collides with nitrogen molecules in the air. These collisions excite the nitrogen atoms, causing them to emit a faint blue glow.

Is radium still used in everyday consumer products?

No, companies stopped using radium in watches and clocks due to radiation hazards. Today, safer materials like non-radioactive phosphors or tritium gas tubes are used instead.

Why is radium so dangerous to human bones?

Radium belongs to the same chemical group as calcium, so the body treats them similarly. If ingested, the body builds radium directly into bones, where it damages bone marrow.

Related terms

Sources & references

By the Chemistry Dictionary editorial team · Reviewed July 17, 2026