Caesium (Cs)
Element 55 · Alkali metal · Solid at 25 °C · Group 1, Period 6
Caesium defines the second: atomic clocks count 9,192,631,770 vibrations of the caesium atom. It is also one of very few metals that melt near room temperature.
On the periodic table, caesium sits in group 1 — the alkali metal group — in period 6. It is one of 6 elements classified as an alkali metal, and at room temperature (25 °C) it is a solid.
The alkali metals are soft, silvery metals with a single electron in their outer shell — which they give up so easily that they are among the most reactive metals known. They react vigorously (sometimes violently) with water, tarnish in air within minutes, and are never found as free metals in nature. In the lab they are stored under oil to keep air and moisture away. Reactivity increases as you move down the group.
Properties of caesium
| Atomic number (Z) | 55 |
|---|---|
| Symbol | Cs |
| Atomic mass | 132.91 |
| Category | Alkali metal |
| Group | 1 |
| Period | 6 |
| Block | s-block |
| Metal / nonmetal | Metal |
| State at 25 °C | Solid |
| Electron configuration | [Xe] 6s1 |
| Electronegativity (Pauling) | 0.79 |
| Melting point | 28.44 °C (301.59 K / 83.19 °F) |
| Boiling point | 671 °C (944.15 K / 1,239.8 °F) |
| Density | 1.873 g/cm³ |
Position in the periodic table
Caesium shares group 1 — the alkali metal group with Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Francium (Fr). Elements in the same group have the same number of outer electrons, so they tend to behave alike chemically — comparing them is one of the fastest ways to see periodic trends in action.
Physical properties
Caesium melts at 28.44 °C (301.59 K / 83.19 °F) and boils at 671 °C (944.15 K / 1,239.8 °F), giving it a liquid range of about 642.6 °C at standard pressure.
Its density is 1.873 g/cm³.
See also: the interactive periodic table, how to read the periodic table, the element families tour.