The most reactive metal, the softest, and the atom that defines the second.
- Atomic number
- 55
- Atomic mass
- 132.905 u
- Phase
- Solid
- Electron configuration
- [Xe] 6s¹
- Melting point
- 301.59 K · 28.4 °C
- Boiling point
- 944 K · 670.9 °C
- Density
- 1.93 g/cm³
- Electronegativity
- 0.79
- Period / Group
- 6 / 1
- Discovery year
- 1860