Barium

Barium (Ba) is an alkaline earth metal with atomic number 56 and an atomic mass of 137.33. Its electron configuration is [Xe] 6s2, which gives it 2 valence electrons.

Barium key data

Symbol	Ba
Atomic number	56
Atomic mass (standard atomic weight)	137.33
Molar mass	137.33 g/mol
Protons	56
Neutrons (most abundant isotope, barium-138)	82
Electrons (neutral atom)	56
Valence electrons	2
Category	Alkaline earth metal
Period	6
Group	2
Block	s-block
Noble gas notation	[Xe] 6s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s2
Electrons per shell	2, 8, 18, 18, 8, 2

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of barium

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

Melting point	1000 K (726.85 °C), 63rd of 103 by melting point
Boiling point	2170 K (1896.9 °C), 55th of 93 by boiling point
Density	3.62 g/cm³, 68th of 96 by density
State at room temperature	Solid
Electronegativity	0.89 (Pauling), 91st of 95 by electronegativity
Van der Waals radius	268 pm, 6th of 99 by atomic radius
First ionisation energy	5.212 eV (502.9 kJ/mol), 97th of 102 by first ionisation energy
Common oxidation states	+2
Discovered	1808

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.

Barium electron configuration

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

Bohr model of barium (Ba): a nucleus and six shells holding 2, 8, 18, 18, 8 and 2 electrons from the inside out. — Bohr model of barium (Ba). 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 barium (Ba), [Xe] 6s2, with the xenon 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 barium (Ba). 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 barium 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 = 137.33 / (6.02214076 × 10²³)
= 2.2804 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 137.33
= 4.3852 × 10²¹ atoms

Barium on the periodic table

Barium is in period 6, group 2, in the s-block.

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