Quick Facts
| Property | Value |
|---|---|
| Symbol | Cs |
| Atomic Number | 55 |
| Atomic Mass | 132.91 u |
| Category | Alkali Metal |
| Period | 6 |
| Group | 1 |
| Block | s |
| Electron Configuration | [Xe] 6s¹ |
Physical Properties
| Property | Value |
|---|---|
| State at 20°C | Solid (barely—melts at 28.5°C) |
| Density | 1.93 g/cm³ |
| Melting Point | 28.5°C (301.7 K) |
| Boiling Point | 671°C (944 K) |
| Appearance | Pale gold, soft metal |
Atomic Properties
| Property | Value |
|---|---|
| Electron Configuration | [Xe] 6s¹ |
| Electronegativity | 0.79 (Pauling scale) |
| First Ionization Energy | 375.7 kJ/mol |
| Atomic Radius | 265 pm |
| Covalent Radius | 244 pm |
| Van der Waals Radius | 343 pm |
History and Discovery
Discovered by: Robert Bunsen and Gustav Kirchhoff Year of Discovery: 1860 Location: Heidelberg, Germany
Etymology
Named from the Latin "caesius" meaning "sky blue," due to the prominent blue lines in its emission spectrum.
Discovery Story
Robert Bunsen and Gustav Kirchhoff discovered cesium in 1860 using their newly invented spectroscope. It was the first element discovered by spectroscopy, identified by two bright blue spectral lines in mineral water from Dürkheim, Germany.
Isotopes
| Isotope | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|
| ¹³³Cs | 100% | Stable | - |
| ¹³⁴Cs | Synthetic | 2.06 years | β⁻ |
| ¹³⁷Cs | Synthetic | 30.17 years | β⁻ |
Occurrence
Natural Abundance
Cesium is rare, comprising about 3 ppm of Earth's crust. It is found in pollucite (the main ore) and lepidolite. Major deposits are in Canada, Zimbabwe, and Namibia.
Extraction and Production
- From Pollucite: Acid digestion and processing
- Electrolysis: Of molten cesium cyanide
- Sources: Primarily pollucite mining
- Global production: about 20 tons annually
Applications and Uses
Atomic Clocks
- Cesium atomic clocks define the second (SI unit of time)
- Most accurate timekeeping devices
- GPS satellites and telecommunications
Oil Drilling
- Cesium formate drilling fluids (high-density, low-damage)
- Deep well drilling
Medical and Research
- Cesium-137 for cancer treatment (brachytherapy)
- Cesium-131 for prostate cancer
- Photoelectric cells
Other Uses
- Photoelectric devices
- Vacuum tubes (getter)
- Infrared detectors
- Propellants (ion engines)
Biological Role
Cesium has no essential biological role.
In the Human Body
Cesium behaves similarly to potassium and can be absorbed. Cesium-137 from nuclear fallout is a health concern as it accumulates in soft tissues.
Health Concerns
Radioactive Cs-137 poses health risks. It was a major contaminant from Chernobyl and Fukushima.
Safety and Hazards
Toxicity
Non-radioactive cesium has moderate toxicity. Radioactive Cs-137 is highly dangerous and can cause radiation sickness and cancer.
Handling Precautions
- Extremely reactive—ignites spontaneously in air
- Reacts explosively with water
- Must be stored under inert gas or vacuum
- Most reactive of all stable elements
Environmental Impact
Cs-137 from nuclear accidents persists in the environment for decades. It contaminates soil, water, and food chains.
Interesting Facts
- Cesium defines the second: exactly 9,192,631,770 oscillations of Cs-133
- It is one of only three metals that are liquid near room temperature
- Cesium has the lowest ionization energy of all elements (except francium)
- It is the most electropositive stable element
- Cesium was the first element discovered by spectroscopy