Cesium vs Francium
Both are alkali metals, so they share the same broad chemistry.
The differences in plain English
- They sit in the same column, group 1, so they have the same number of outer electrons and react in similar ways; the heavier one, Francium, is usually the more reactive of the two if they are metals, and the less reactive if they are non-metals.
- Both are solids at room temperature.
- Cesium is denser, only slightly more than Francium (1.93 against 1.87 g/cm³).
- Cesium melts at 29 °C, Francium at 27 °C.
- An atom of Francium is about 68 per cent heavier than an atom of Cesium (223.00 u against 132.91 u).
- Francium pulls electrons more strongly in a bond (electronegativity 0.79 against 0.79).
Side by side
| Cesium | Francium | |
|---|---|---|
| Atomic number | 55 | 87 |
| Symbol | Cs | Fr |
| Family | Alkali metal | Alkali metal |
| Group / period | 1 / 6 | 1 / 7 |
| State at room temperature | Solid | Solid |
| Atomic mass | 132.905 u | 223.000 u |
| Density | 1.93 g/cm³ | 1.87 g/cm³ |
| Melting point | 29 °C | 27 °C |
| Boiling point | 671 °C | 677 °C |
| Electronegativity | 0.79 | 0.79 |
| First ionisation energy | 375.7 kJ/mol | 380 kJ/mol |
| Electron configuration | [Xe] 6s1 | [Rn] 7s1 |
| Discovered by | Robert Bunsen | Marguerite Perey |
Cesium
Caesium, spelt cesium in American English and in the data behind this site, is the most reactive of all the stable metals. Drop it into water and it explodes. It melts at 28.5 °C, so on a hot summer day a sealed ampoule of it turns to a pale golden liquid in your hand. Yet this alarming element has one of the most orderly jobs imaginable: it defines the second.
- The second is defined as exactly 9,192,631,770 oscillations of the microwave radiation that flips caesium-133 atoms between two energy states.
- Caesium was the first element discovered using a spectroscope, and it was named after the sky-blue lines it produces.
Francium
Francium sits at the bottom of the alkali metals, beneath lithium, sodium and caesium, and on paper it should be the most reactive metal of all. In practice nobody has ever tested that, because nobody has ever had a visible piece. The most stable isotope, francium-223, loses half its atoms every 22 minutes.
- Marguerite Perey joined the Radium Institute at nineteen as Marie Curie's personal laboratory assistant and discovered francium ten years later.
- The total amount of francium in the Earth's crust at any moment is estimated at only 20 to 30 grams.
Colour the bubbles by density or melting point and the difference jumps out.
More comparisons
Data from Periodic-Table-JSON (CC BY-SA); temperatures rounded to the nearest degree. Written by Anthony.