Cesium (Cs)

The second is defined as exactly 9,192,631,770 oscillations of the radiation from a caesium-133 hyperfine transition, so every clock on Earth, every GPS fix and every financial timestamp is ultimately counting caesium. It is the most reactive metal in practical terms — it ignites on contact with air and reacts explosively with water, even with ice at −116 °C — and has the lowest electronegativity of any stable element at 0.79. It is also faintly golden, one of only a handful of coloured metals alongside gold and copper.

Properties

Atomic number

55

Symbol

Cs

Standard atomic weight

132.91 u

Category

Alkali metal

Group

1

Period

6

Block

s-block

Electron configuration

[Xe] 6s¹

Electrons per shell

2, 8, 18, 18, 8, 1

Melting point

301.59 K — 28.4 °C — 83.2 °F

Boiling point

944 K — 670.9 °C — 1239.5 °F

Density

1.93 g/cm³

Electronegativity

0.79 (Pauling)

Oxidation states

+1

Discovered

1860 by Robert Bunsen and Gustav Kirchhoff

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹

Noble gas shorthand

[Xe] 6s¹

Shells

K: 2 · L: 8 · M: 18 · N: 18 · O: 8 · P: 1

What it is used for

• Caesium atomic clocks, which define the second
• Dense caesium formate drilling fluids for deep oil wells
• Photoelectric cells and infrared night-vision photocathodes
• Caesium-137 gamma sources for industrial gauges and sterilisation
• Getters that scavenge residual gas in vacuum tubes

Hazards

• Ignites spontaneously in air; reacts explosively with water including ice
• Caesium-137 is a major fallout and reactor-accident contaminant with a 30-year half-life

Spelled "cesium" here following US usage and IUPAC's accepted alternative; "caesium" is the primary IUPAC spelling and is carried as an alias. Melts at 28.4 °C, so like gallium it liquefies in a warm hand — through the sealed ampoule it has to be kept in.

Neighbours in the table

Group 1

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Cesium sits in group 1 with hydrogen, lithium, sodium, potassium, rubidium, francium.