Radium

Radium (Ra) is an alkaline earth metal with atomic number 88. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight; its longest-lived isotope has mass number 226. Its electron configuration is [Rn] 7s2, which gives it 2 valence electrons.

Radium key data

Symbol	Ra
Atomic number	88
Mass number (longest-lived isotope)	226
Protons	88
Neutrons (longest-lived isotope, radium-226)	138
Electrons (neutral atom)	88
Valence electrons	2
Category	Alkaline earth metal
Period	7
Group	2
Block	s-block
Noble gas notation	[Rn] 7s2
Full electron configuration	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

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

Measured properties of radium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown.

Melting point	973 K (699.85 °C), 64th of 103 by melting point
Boiling point	1413 K (1139.9 °C), 66th of 93 by boiling point
Density	5 g/cm³, 63rd of 96 by density
State at room temperature	Solid
Electronegativity	0.9 (Pauling), 90th of 95 by electronegativity
Van der Waals radius	283 pm, 4th of 99 by atomic radius
First ionisation energy	5.279 eV (509.3 kJ/mol), 96th of 102 by first ionisation energy
Common oxidation states	+2
Discovered	1898

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Radium electron configuration

The full electron configuration of radium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 7s2. With the noble-gas core collapsed it is [Rn] 7s2, and the outermost subshell is 7s2.

Bohr model of radium (Ra): a nucleus and seven shells holding 2, 8, 18, 32, 18, 8 and 2 electrons from the inside out. — Bohr model of radium (Ra). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of radium (Ra), [Rn] 7s2, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing every orbital drawn holds a pair of electrons. — Orbital diagram of radium (Ra). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one radium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 226 / (6.02214076 × 10²³)
= 3.7528 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 226
= 2.6647 × 10²¹ atoms

Radium has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of radium-226, so treat these as nominal.

Radium on the periodic table

Radium is in period 7, group 2, in the s-block.

The other elements of group 2 are beryllium, magnesium, calcium, strontium and barium.