Chemical element · reference page

55Cs

Caesium

Caesium is element 55 (Cs), located in period 6, group 1 and the s block. At room temperature its reference phase is solid. Its electron configuration is [Xe] 6s1.

Atomic number55

Standard atomic weight132.90545196(6)

Group1

Period6

Blocks

Electron configuration[Xe] 6s1

Electronic structure

What controls its chemistry

The outer electron configuration ends in s¹. Losing that single valence electron gives a +1 ion with a filled inner-shell configuration, so cesium compounds are dominated by the +1 oxidation state.

The same outer-shell pattern is shared by the alkali metals. Down the group the outer electron is farther from the nucleus and generally easier to remove, which helps explain the high reactivity of the family.

Chemical forms

Compounds, ions and materials

The dominant chemical form is cesium in the +1 oxidation state. Ionic halides, hydroxides and oxygen-containing salts are common; the free metal is much less common in nature because it reacts readily with water, oxygen and other non-metals.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 1.93 g/cm³. The melting point is 301.7 K (28.5 °C). The boiling point is 944 K (670.9 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where caesium is found and used

In practice, caesium is encountered in atomic clocks, drilling fluids and scientific research.

In practice

Examples of real uses

Atomic clocks

Atomic transitions in caesium provide an extremely stable frequency reference for precision timekeeping.

Drilling fluids

Caesium compounds with suitable density or chemistry are added to drilling fluids to control pressure and fluid properties in wells.

Scientific research

Caesium is produced, isolated or measured mainly to test atomic, nuclear and periodic trends rather than for bulk commercial use.

Periodic position

What its neighbours tell us

Caesium is in period 6 and group 1. In the neighbourhood shown here, barium is immediately to its right in the same period; rubidium is above it and francium below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationalkali metal

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 18 · 8 · 1

Density1.93 g/cm³

Melting point301.7 K (28.5 °C)

Boiling point944 K (670.9 °C)

Pauling electronegativity0.79

First ionisation energy375.7 kJ/mol

Electron affinity45.51 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of caesium is associated with Robert Bunsen. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 132.90545196(6). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Cesium has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

Elemental cesium reacts readily with moisture and can react violently with water. It is handled away from water and air under controlled conditions.