Caesium
Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium is an alkali metal and has physical and chemical properties similar to those of rubidium and potassium.
At a glance
- Atomic number
- 55
- Atomic mass
- 132.905452 u
- Phase
- Solid
- Density
- 1.93 g/cm³
- Melting point
- 301.7 K
- Boiling point
- 944 K
- First ionization energy
- 375.7 kJ/mol
- Electronegativity
- 0.79
- Electron affinity
- 45.505 kJ/mol
- Molar heat capacity
- 32.21 J/(mol·K)
- Period
- 6
- Appearance
- silvery gold
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Caesium’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
54 is the [Xe] core total across five shells.
Preparing the orbital view…
55 protons78 neutrons
Caesium-133: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid
At this temperature, caesium is a solid.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
CsNeutral atom · no charge
A neutral caesium atom. Removing its first electron takes 376 kJ/mol.
Caesium’s outer shell holds 1 electron. Electron 2 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 5.9× as much energy as the one before. That’s why caesium forms Cs⁺ ions. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 1, then 8, then 18, then 18, then 8, then 2 bars. Read backwards, that’s 2, 8, 18, 18, 8, 1: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The single electron before the first big jump is caesium’s valence electron, the one it uses in chemical reactions. That’s why caesium sits in group 1: every element in the group has the same number of outer electrons, so they react in similar ways.
- The numbers explain the charge. Removing electron 2 as well, to make Cs²⁺, would cost 2,234 kJ/mol. Forming a compound gives back far less energy than that, so Cs²⁺ doesn’t form in ordinary chemistry.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 6 | 376 | – | Measured |
| 2 | 5 | 2,234 | 5.9 | Measured |
| 3 | 5 | 3,203 | 1.4 | Estimated |
| 4 | 5 | 4,149 | 1.3 | Calculated |
| 5 | 5 | 5,403 | 1.3 | Calculated |
| 6 | 5 | 6,667 | 1.2 | Calculated |
| 7 | 5 | 7,999 | 1.2 | Calculated |
| 8 | 5 | 10,623 | 1.3 | Estimated |
| 9 | 5 | 12,120 | 1.1 | Estimated |
| 10 | 4 | 20,580 | 1.7 | Measured |
| 11 | 4 | 22,481 | 1.1 | Calculated |
| 12 | 4 | 25,183 | 1.1 | Calculated |
| 13 | 4 | 27,884 | 1.1 | Calculated |
| 14 | 4 | 30,489 | 1.1 | Calculated |
| 15 | 4 | 33,963 | 1.1 | Calculated |
| 16 | 4 | 36,857 | 1.1 | Calculated |
| 17 | 4 | 39,848 | 1.1 | Calculated |
| 18 | 4 | 42,936 | 1.1 | Calculated |
| 19 | 4 | 45,927 | 1.1 | Calculated |
| 20 | 4 | 57,602 | 1.3 | Calculated |
| 21 | 4 | 60,689 | 1.1 | Calculated |
| 22 | 4 | 64,259 | 1.1 | Calculated |
| 23 | 4 | 67,540 | 1.1 | Calculated |
| 24 | 4 | 72,653 | 1.1 | Calculated |
| 25 | 4 | 76,320 | 1.1 | Calculated |
| 26 | 4 | 84,425 | 1.1 | Calculated |
| 27 | 4 | 88,390 | 1 | Estimated |
| 28 | 3 | 153,605 | 1.7 | Calculated |
| 29 | 3 | 161,323 | 1.1 | Calculated |
| 30 | 3 | 169,525 | 1.1 | Calculated |
| 31 | 3 | 178,305 | 1.1 | Calculated |
| 32 | 3 | 186,796 | 1 | Calculated |
| 33 | 3 | 195,769 | 1 | Calculated |
| 34 | 3 | 206,189 | 1.1 | Calculated |
| 35 | 3 | 215,162 | 1 | Calculated |
| 36 | 3 | 224,714 | 1 | Calculated |
| 37 | 3 | 233,687 | 1 | Calculated |
| 38 | 3 | 258,870 | 1.1 | Calculated |
| 39 | 3 | 266,975 | 1 | Calculated |
| 40 | 3 | 275,852 | 1 | Calculated |
| 41 | 3 | 284,149 | 1 | Calculated |
| 42 | 3 | 300,841 | 1.1 | Calculated |
| 43 | 3 | 310,104 | 1 | Calculated |
| 44 | 3 | 327,278 | 1.1 | Calculated |
| 45 | 3 | 336,251 | 1 | Calculated |
| 46 | 2 | 770,821 | 2.3 | Calculated |
| 47 | 2 | 793,495 | 1 | Calculated |
| 48 | 2 | 818,582 | 1 | Calculated |
| 49 | 2 | 841,931 | 1 | Calculated |
| 50 | 2 | 902,138 | 1.1 | Calculated |
| 51 | 2 | 929,057 | 1 | Calculated |
| 52 | 2 | 962,345 | 1 | Calculated |
| 53 | 2 | 984,998 | 1 | Calculated |
| 54 | 1 | 4,038,980 | 4.1 | Calculated |
| 55 | 1 | 4,140,489 | 1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
Drop it in water
Ready to drop a small piece of caesium.
Explodes the moment it touches the water, even when the water is ice-cold.
- In water it
- Sinks1.93 g/cm³, water is 1
- From the heat it
- Melts into a ballmelts at 29 °C
- How violent
2Cs + 2H₂O → 2CsOH + H₂
Hydrogen gas fizzes off, and the metal hydroxide dissolves, making the water alkaline: a few drops of phenolphthalein indicator turn it pink.
Why it gets more violent down the group
Every alkali metal has one electron in its outer shell, and reacting means giving it away. Further down the group that electron sits in a shell further from the nucleus, shielded by more electrons inside, so it takes less energy to remove and the reaction runs faster.
- Lithium520 kJ/mol
- Sodium496 kJ/mol
- Potassium419 kJ/mol
- Rubidium403 kJ/mol
- Caesium376 kJ/mol
Energy to remove the outer electron (first ionization energy, NIST). Speed isn’t the same as energy: per atom, lithium’s reaction releases slightly more energy than sodium’s, it just releases it slowly. Francium’s value (393 kJ/mol) is a little higher than caesium’s, because of relativistic effects, so it may not be the most violent after all.
Don’t try this at home. Demonstrations use rice-grain-sized pieces behind safety screens; the heavier metals are only shown on video.
How does it compare?
Density
1.93 g/cm³22nd lowest of 99
Melting point
302 K14th lowest of 96
Boiling point
944 K18th lowest of 97
Electronegativity
0.792nd lowest of 96
First ionization energy
376 kJ/molThe easiest to ionize of 99 elements
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- CaesiumRoyal Society of Chemistry
- CaesiumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database