A cesium atom is the smallest unit of cesium, a soft, silvery metal that melts at 28.5 degrees Celsius (83 degrees Fahrenheit). It is about 0.5 nanometers (500 picometers) across, measured as twice its covalent radius. That makes it one of the largest atoms of any element. A nanometer is a billionth of a meter, and a picometer is a thousandth of a nanometer.

Size

An atom has no hard edge. Its electrons form a cloud around the nucleus that thins out gradually, so chemists measure an atom's size in several ways. The covalent radius is half the distance between the centers of two identical atoms joined by a chemical bond. For cesium it is 244 picometers, so the atom is about 0.49 nanometers across.

Other ways of measuring give cesium a radius of 260–343 picometers, which makes it 0.52–0.69 nanometers across. In PubChem's tables of atomic radii, no element has larger atoms except francium. Francium is so radioactive that even its longest-lived form loses half its atoms in 22 minutes, and no one has collected enough of it to weigh.

Atoms get larger down each column of the periodic table, because each row lower down adds electrons farther from the nucleus. Cesium sits near the bottom of the first column. Its outermost electron travels farther from the nucleus than any of its other 54 electrons.

Size comparisons

About 160,000–200,000 cesium atoms side by side would span a human hair, which is 0.08–0.1 millimeters wide.
Measured the same way, by covalent radius, a cesium atom is about three times as wide as a carbon atom.
A cesium atom is not quite twice as wide as a water molecule, which is about 0.28 nanometers across.
The microwaves that cesium atoms respond to in an atomic clock have a wavelength of about 3.3 centimeters (1.3 inches), about 67 million times the width of the atom.

How cesium defines the second

All natural cesium is cesium-133, its only stable form. An atom of cesium-133 can flip between two energy states when it absorbs microwaves of one exact frequency. The second, the international unit of time, is defined from that frequency. One second is the time taken by 9,192,631,770 cycles of those microwaves.

Atomic clocks use this definition to keep time. In a fountain clock at the National Institute of Standards and Technology (NIST) in the United States, lasers cool a cloud of about 100,000 cesium atoms until they are nearly still. The clock then tosses the cloud about a meter (3 feet) upward.

Microwaves pass through the atoms as they rise and fall, and the clock tunes the microwaves until their frequency matches the one the atoms absorb. As of 2026, the best of these clocks are accurate to within one second in 100 million years.

Seeing cesium

The German chemist Robert Bunsen and the German physicist Gustav Kirchhoff discovered cesium in 1860, in mineral water from Dürkheim. They found it with a spectroscope, an instrument that spreads light into its separate colors. They named it after the Latin word caesius, "sky blue", for the bright blue lines in its light.

No one can see a single cesium atom through an ordinary microscope, because the wavelength of light limits the smallest detail a light microscope can show. In 1981, Gerd Binnig and Heinrich Rohrer built the scanning tunneling microscope. It passes an extremely fine point very close to a surface and makes images in which single atoms can be picked out.

Sources

Cesium. PubChem, National Library of Medicine.
Carbon. PubChem, National Library of Medicine.
Cesium. Periodic Table of Elements, Los Alamos National Laboratory.
Francium. Periodic Table of Elements, Los Alamos National Laboratory.
Periodic Variations in Element Properties. In Chemistry 2e. OpenStax, 2019.
SI base unit: second (s). International Bureau of Weights and Measures (BIPM).
Fountains of Atoms: Exquisite Timekeepers. National Institute of Standards and Technology, 2025.
Gerd Binnig: Facts. Nobel Prize Outreach.
Speed of light in vacuum. NIST CODATA.
Benchmark oxygen-oxygen pair-distribution function of ambient water from x-ray diffraction measurements with a wide Q-range. Skinner et al., Journal of Chemical Physics, 2013.
Just How Small Is "Nano"?. National Nanotechnology Initiative.

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