55CsCaesium132.91

A real sample of the element Caesium — Real sample photo

Caesium (Cs)

“The metal that melts in the palm of your hand — and whose atoms now define, vibration by vibration, every second the world lives by.”

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Atomic number55

Atomic mass132.91

Group1

Period6

BlockS-block

PhaseSolid

Electron structure

How its electrons are arranged

[Xe] 6s¹

Shell 1

2e⁻

Shell 2

8e⁻

Shell 3

18e⁻

Shell 4

18e⁻

Shell 5

8e⁻

Shell 6

1e⁻

Computed from the aufbau principle, with known real-world exceptions applied automatically.

🎯 Exam focus

Questions to practise

Try each question yourself first — then open it to read the full model answer.

1Which pair of Heidelberg scientists found caesium in 1860 with a brand-new optical instrument, and what exactly did they see through it?

Model answerThe pair were Robert Bunsen and Gustav Kirchhoff, working at Heidelberg, who in 1860 turned their newly invented spectroscope on the mineral water of Dürkheim. Through it they saw two brilliant sky-blue lines belonging to no known element — the fingerprint of caesium, the first element ever discovered by spectroscopy. The scattered metal itself stayed unseen until 1882.

2Explain how caesium-133 atoms define the SI second, and why no pendulum or quartz crystal could hold the job as steadily.

Model answerThe SI second is defined as exactly 9,192,631,770 vibrations of the microwave transition of a caesium-133 atom, because every such atom vibrates at one identical, unchanging frequency fixed by nature. Pendulums swing unevenly and quartz crystals drift with temperature and age, while the best caesium clocks would not drift a second in a hundred million years.

3Name two everyday technologies in Bangladesh whose timing secretly leans on caesium-trained clocks.

Model answerFirst, the map app that tracks a CNG or bus through Dhaka: GPS positioning depends on satellite signals whose timestamps are disciplined by caesium clocks. Second, the mobile phone network: every tower’s signal handshake stays in perfect step thanks to caesium-trained timing, so calls and data stay in order across the country.

4A lump of caesium would melt in a bare fist — yet no one dares hold it that way. Explain the melting point and the danger together, using its behaviour with water.

Model answerCaesium melts at just 28.5 °C, so the warmth of a palm — a hot Dhaka afternoon will do — slowly turns a lump liquid. But the same metal explodes on meeting water, even ice a hundred degrees below zero, skating across the surface as hydrogen bursts into flame. Sweat on bare skin could set off the same violence, so it stays sealed under argon or oil.

5Build caesium’s electron configuration outward from the xenon core, and use it to explain why Group 1 reactivity reaches its climax at this bottom member.

Model answerXenon carries 54 electrons; caesium, the 55th, adds one electron to the new sixth shell: [Xe] 6s¹. That lone valence electron sits far from the nucleus, shielded by 54 inner electrons, giving caesium the lowest ionisation energy of all stable elements. Since it surrenders its outer electron most eagerly, Group 1 reactivity peaks at this bottom member.

6What does “caesium” mean in Latin, and how did the element win a colour-based name years before anyone had set eyes on the metal itself?

Model answerCaesius is Latin for “sky-blue”. The element won its name from colour alone: Bunsen and Kirchhoff’s spectroscope showed two brilliant blue lines in Dürkheim mineral water that belonged to no known element, and that sky-blue signature named the metal in 1860. Bunsen never once saw it pure; the first visible golden drops only came in 1882, when Carl Setterberg electrolysed molten caesium cyanide.

Quick answers

One fact, one page

Every key fact of this element has its own clean page — a direct two-line answer first, then the working. Fast to load, easy to share.

Electron configurationThe full arrangement, step by step ValencyHow many bonds it forms Atomic massThe standard weight, explained Metal or nonmetalWhich family it belongs to Everyday usesWhere you meet it daily

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The notes

About the element

Characteristics

Electron configuration [Xe] 6s¹ — a single, loosely held valence electron giving the lowest ionisation energy of all stable elements; Period 6, Group 1, heavyweight of the alkali metals.
Frightfully reactive: it explodes on meeting water — even ice more than a hundred degrees below zero — skating across the surface as the hydrogen around it bursts into flame.
Melts at 28.5 °C, the heat of a warm Dhaka afternoon; held in a sealed vial, a lump slowly goes liquid from the warmth of your palm alone.
Pale gold rather than silvery — one of only three metals, with gold and copper, that absorb blue light instead of reflecting it all.

Appearance

The softest metal of all — a pale golden block that dents under a spoon, and one of only a handful of metals liquid near room temperature, melting at just 28.5 °C. Dry air alone sets it burning, so it lives sealed in glass ampoules under argon or oil.

Real world

Where you meet it

✦ Uses

The definition of time itself: 9,192,631,770 vibrations of a caesium-133 atom’s microwave transition make one SI second, and the best caesium clocks would not drift a second in a hundred million years.
Every GPS satellite and telecom network leans on caesium-trained clocks, so that a map app on a Dhaka bus and every phone tower’s signal handshake stay in perfect step.
Caesium formate brines — extraordinarily dense yet kind to equipment — stabilise scorching, high-pressure oil and gas wells during deep drilling.
Photocathodes that spill electrons at the merest whisper of light: caesium surfaces sit inside night-vision tubes, photomultipliers and old television camera tubes.

◍ Everyday examples

The “second” printed in your physics textbook is defined by neither the Sun nor a pendulum but by caesium-133 — the only element that owns an SI unit.
The map app that tracks a CNG or bus through Dhaka positions itself using satellite time that is ultimately disciplined by caesium standards.
Wildlife documentaries’ grainy green night footage is often captured through tubes whose caesium-coated surfaces can see almost in the dark.

Story

History & name

In 1860 Robert Bunsen and Gustav Kirchhoff in Heidelberg pointed their brand-new spectroscope at the mineral water of Dürkheim and caught two bright sky-blue lines belonging to no known element — making caesium the first element ever discovered by spectroscopy. They named it from the Latin caesius, “sky-blue”. So scattered was the metal that Bunsen never once saw it pure; only in 1882 did Carl Setterberg, electrolysing molten caesium cyanide, finally pry out the first visible drops of the golden metal.

Name originLatin caesius, “sky-blue” — for the pair of brilliant blue spectral lines that betrayed the element in 1860.

Neighbours

Nearby elements

The elements that sit right beside this one on the table — often the closest chemical cousins.

53IIodine126.90 54XeXenon131.29 56BaBarium137.33 57LaLanthanum138.91