Radium (Ra)
Element 88 · Alkaline earth metal · atomic mass [226] u · group 2, period 7
Electron shells: 2, 8, 18, 32, 18, 8, 2
Radium glows faintly blue in the dark and was once marketed as a health tonic. The women who painted it onto watch dials paid a horrific price for that mistake.
The "Radium Girls" who licked their brushes to a point sued their employer and reshaped US labour law.
- Atomic number
- 88
- Atomic mass
- [226] u
- Electron configuration
- [Rn] 7s²
- Group · period · block
- 2 · 7 · s
- Oxidation states
- +2
- Electronegativity
- 0.9 (Pauling)
- Melting point
- 700 °C
- Boiling point
- 1,140 °C
- Density
- 5 g/cm³
- State at 20 °C
- Solid
- Appearance
- Silvery-white metal
- Discovered
- Marie and Pierre Curie, 1898
88 protons, and 88 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Alkaline earth metals
What Radium is used for
- Historically, luminous dials and radiotherapy
- Radiation-source research
Atomic structure of Radium
A neutral radium atom has 88 protons and 88 electrons. Its most common isotope carries about 138 neutrons, which is where the mass number 226 comes from. Change the neutron count and you have a different isotope of radium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 18, 8, 2. In orbital notation that is [Rn] 7s², leaving 2 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Radium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Radium sits in the periodic table
Discovery and naming
Radium was discovered by Marie and Pierre Curie in 1898. Its name comes from Latin radius, "ray".
Every isotope of radium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Radium?
Radium has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 226 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Radium have?
Radium has 88 protons, because the atomic number is the proton count and that is what makes an atom Radium at all. A neutral Radium atom has the same number of electrons, 88. The neutron count varies by isotope; the most common one has about 138, since 226 − 88 = 138.
What is the electron configuration of Radium?
The electron configuration of Radium is [Rn] 7s². Its electrons are arranged 2, 8, 18, 32, 18, 8, 2 across 7 shells.
Is Radium a metal?
Yes. Radium is classified as a alkaline earth metal, on the left side of the table.
What group and period is Radium in?
Radium is in group 2 (the alkaline earth metals), period 7, in the s-block. The group is its column and the period is its row.
What is the charge of Radium?
Radium most commonly forms a 2+ ion. It has 2 electrons in its outer shell and loses them readily, which is what the group number tells you.
What is the melting point of Radium?
Radium melts at 700 °C and boils at 1,140 °C. That puts it in the solids at everyday temperatures.
What is Radium used for?
Historically, luminous dials and radiotherapy, Radiation-source research. Radium glows faintly blue in the dark and was once marketed as a health tonic.
Who discovered Radium, and where does its name come from?
Radium (Ra) was discovered by Marie and Pierre Curie in 1898. The name comes from Latin radius, "ray".
Is Radium radioactive?
Yes. Every isotope of Radium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.