Barium at a glance
- Atomic number
- 56
- Symbol
- Ba
- Atomic mass
- 137.33 u
- Category
- Alkaline earth
- Group / period
- 2 / 6
- Electron configuration
- [Xe] 6s 2
- State at 25 °C
- Solid
- Melting point
- 726.9 °C
- Boiling point
- 1897 °C
- Density
- 3.594 g/cm³
- Electronegativity
- 0.89
- 1st ionization energy
- 503 kJ/mol
- Oxidation states
- +2
- Discovered
- 1808
Atomic structure
A neutral barium atom has 56 protons and 56 electrons, and its most common isotope carries about 82 neutrons. One atom weighs about 2.28 × 10⁻²⁵ kg, so a mole of barium (6.022 × 10²³ atoms) weighs 137.33 g. That nucleus holds 1.46 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 6 shells (K 2, L 8, M 18, N 18, O 8, P 2), which gives the configuration [Xe] 6s 2. With 2 electrons in its outer shell, it tends to lose electrons and form positive ions. It has 7 stable isotopes: Ba-130, Ba-132, Ba-134, Ba-135, Ba-136, Ba-137 and Ba-138. Its most common isotope, Ba-138, has the same mass number as Ce-138: isobars, nuclei of different elements with 138 nucleons each.
Physical properties
At room temperature (25 °C) barium is a solid. It melts at 726.9 °C and boils at 1897 °C, so it stays liquid across a 1170-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 3.594 g/cm³, 3.59 times as dense as water. Its atoms have a radius of about 215 pm (Caesium next door is 260 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 0.89 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 503 kJ/mol, less than Strontium above it (550 kJ/mol). A bond between barium and chlorine would be mostly ionic, with an electronegativity difference of 2.3. It releases 14 kJ/mol when it gains an extra electron. Its common oxidation states are +2, so in compounds you will usually meet it as Ba²⁺. Group 2 also holds Beryllium, Magnesium, Calcium, Strontium and Radium, which share its outer-shell pattern. Its Lewis symbol is Ba surrounded by 2 dots. Handle with care: as a pure element it is toxic and flammable or reacts with water.
Where it comes from
It makes up 425 mg/kg of Earth's crust, the 14th most abundant element there. In the human body it accounts for about 0.00003 % of body mass. It was first identified in 1808, making it the 46th element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 553.5 nm, 455.4 nm and 493.4 nm. In a flame test its compounds colour the flame pale apple green.