Caesium at a glance
- Atomic number
- 55
- Symbol
- Cs
- Atomic mass
- 132.91 u
- Category
- Alkali metal
- Group / period
- 1 / 6
- Electron configuration
- [Xe] 6s 1
- State at 25 °C
- Solid
- Melting point
- 28.44 °C
- Boiling point
- 670.9 °C
- Density
- 1.873 g/cm³
- Electronegativity
- 0.79
- 1st ionization energy
- 376 kJ/mol
- Oxidation states
- +1
- Discovered
- 1860
Atomic structure
A neutral caesium atom has 55 protons and 55 electrons, and its most common isotope carries about 78 neutrons. One atom weighs about 2.21 × 10⁻²⁵ kg, so a mole of caesium (6.022 × 10²³ atoms) weighs 132.91 g. That nucleus holds 1.42 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 1), which gives the configuration [Xe] 6s 1. With 1 electron in its outer shell, it tends to lose electrons and form positive ions. It has only one stable isotope, Cs-133. No other element has a stable isotope with mass number 133, so Cs-133 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) caesium is a solid. It melts at 28.44 °C and boils at 670.9 °C, so it stays liquid across a 642.4-degree range. It would melt in your hand (body temperature). Its density is 1.873 g/cm³, 1.87 times as dense as water. Its atoms have a radius of about 260 pm (Barium next door is 215 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 0.79 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 376 kJ/mol, less than Rubidium above it (403 kJ/mol). A bond between caesium and chlorine would be mostly ionic, with an electronegativity difference of 2.4. It releases 46 kJ/mol when it gains an extra electron. Its common oxidation states are +1, so in compounds you will usually meet it as Cs⁺. Group 1 also holds Hydrogen, Lithium, Sodium, Potassium, Rubidium and Francium, which share its outer-shell pattern. Its Lewis symbol is Cs surrounded by 1 dot. Handle with care: as a pure element it is flammable or reacts with water.
Where it comes from
It makes up 3 mg/kg of Earth's crust, the 46th most abundant element there. It was first identified in 1860, making it the 59th 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 455.5 nm, 459.3 nm and 672.3 nm. In a flame test its compounds colour the flame blue-violet.