Alkaline earth metalElement 88 · s-block

Radium (Ra)

Radium (Ra) is an alkaline earth metal with atomic number 88; its most stable known isotope has mass number 226. At room temperature it is a solid that melts at 696 °C and boils at 1,737 °C.

88RaRadium[226]

Atomic number

protons

Atomic weight

[226]

most stable isotope

Melting point

696 °C

969.15 K

Boiling point

1,737 °C

2,010 K

Density

5 g/cm³

near room temperature

Electronegativity

0.9

Pauling scale

State at 25 °C

Solid

Discovered

1898

France

Radium at a glance

Identity

Symbol	Ra
Atomic number (Z)	88
Atomic weight	[226] mass number of the most stable isotope
Category	Alkaline earth metal
Group	2 — alkaline earth metals
Period	7
Block	s-block
CAS number	7440-14-4

Atomic properties

Electron configuration	[Rn] 7s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 7s²
Electrons per shell	2, 8, 18, 32, 18, 8, 2
Oxidation states	+2
Electronegativity (Pauling)	0.9
First ionisation energy	5.278 eV (509.3 kJ/mol) 2nd: 10.147 eV · 3rd: 31 eV
Electron affinity	0.1 eV
Atomic radius (empirical)	215 pm
Covalent radius	201 pm
Van der Waals radius	283 pm

Physical properties

State at 25 °C	Solid
Melting point	696 °C · 1,284.8 °F · 969.15 K
Boiling point	1,736.8 °C · 3,158.3 °F · 2,010 K
Density	5 g/cm³

Isotopes & abundance

Natural occurrence	From radioactive decay
Radioactive	Yes
Most stable isotope	²²⁶Ra — half-life 1.6 thousand years
Known isotopes	35
Abundance in Earth's crust	0.000001 mg/kg
Abundance in seawater	0 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Radium electron configuration

Radium has 88 electrons. Written in short form, its ground-state configuration is [Rn] 7s² — the electrons of the noble gas radon, plus 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 7s².

As a member of group 2 it has 2 valence electrons in its outer shell (shell 7), which largely decide its chemistry.

7s↑↓

Discovery and name

Discovered

1898 — Marie and Pierre Curie · France.

Name

Latin radius, ray.

Known for

Once used in luminous watch dials, which harmed the workers who painted them. Radium-223 is used to treat prostate cancer that has spread to bone.

Isotopes of radium

Radium has no stable isotopes. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
²²⁶Ra	88	138	1.6 thousand years

35 isotopes of radium are known in our data set (half-lives as published in nuclear data tables).

Radium compared with group 2

The other elements in group 2 (alkaline earth metals), from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Beryllium	4	9.0122	1,287 °C	2,468 °C	1.85 g/cm³	1.57
Magnesium	12	24.305	650 °C	1,090 °C	1.74 g/cm³	1.31
Calcium	20	40.078	842 °C	1,484 °C	1.54 g/cm³	1
Strontium	38	87.62	777 °C	1,377 °C	2.64 g/cm³	0.95
Barium	56	137.33	727 °C	1,845 °C	3.51 g/cm³	0.89
Radium	88	[226]	696 °C	1,737 °C	5 g/cm³	0.9

Frequently asked questions

6 answers

What is the atomic mass of radium?

Radium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [226], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does radium have?

A neutral radium atom has 88 protons and 88 electrons. Its most stable isotope, radium-226, has 138 neutrons.

What is the electron configuration of radium?

[Rn] 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 7s²).

What are the melting and boiling points of radium?

Radium melts at 696 °C (1,284.8 °F) and boils at 1,736.85 °C (3,158.3 °F).

Is radium a metal?

Radium is a metal. It is in group 2, period 7, and is a solid at room temperature.

Who discovered radium?

1898 — Marie and Pierre Curie (France).