Francium at a glance
- Atomic number
- 87
- Symbol
- Fr
- Atomic mass
- 223 u
- Category
- Alkali metal
- Group / period
- 1 / 7
- Electron configuration
- [Rn] 7s 1
- State at 25 °C
- Solid
- Melting point
- 26.85 °C
- Boiling point
- 676.9 °C
- Density
- 1.87 g/cm³
- Electronegativity
- 0.7
- 1st ionization energy
- 393 kJ/mol
- Oxidation states
- +1
- Discovered
- 1939
Atomic structure
A neutral francium atom has 87 protons and 87 electrons, and its most common isotope carries about 136 neutrons. One atom weighs about 3.7 × 10⁻²⁵ kg, so a mole of francium (6.022 × 10²³ atoms) weighs 223 g. That nucleus holds 1.56 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 7 shells (K 2, L 8, M 18, N 32, O 18, P 8, Q 1), which gives the configuration [Rn] 7s 1. With 1 electron in its outer shell, it tends to lose electrons and form positive ions. All of its isotopes are radioactive, so any francium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) francium is a solid. It melts at 26.85 °C and boils at 676.9 °C, so it stays liquid across a 650-degree range. It would melt in your hand (body temperature). Its density is 1.87 g/cm³, 1.87 times as dense as water.
Chemical behaviour
Its electronegativity is 0.7 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 393 kJ/mol, more than Caesium above it (376 kJ/mol). A bond between francium and chlorine would be mostly ionic, with an electronegativity difference of 2.5. It releases 47 kJ/mol when it gains an extra electron. Its common oxidation states are +1, so in compounds you will usually meet it as Fr⁺. Group 1 also holds Hydrogen, Lithium, Sodium, Potassium, Rubidium and Caesium, which share its outer-shell pattern. Its Lewis symbol is Fr surrounded by 1 dot. Handle with care: as a pure element it is radioactive and flammable or reacts with water.
Where it comes from
It is vanishingly rare in nature, existing only briefly as part of radioactive decay chains. It was first identified in 1939, making it the 90th element to be discovered (counting the 11 known since antiquity together).