ElementsWhat are the alkali metals?
The alkali metals fill the first column of the table, below hydrogen. They are soft enough to cut with a knife, shiny when freshly cut, and light. Each atom has one outer electron that it gives up very easily, which makes these the most reactive metals. All of them react with water to give hydrogen gas and an alkali, and the reaction gets more violent going down the column. For this reason they are never found pure in nature and are stored under oil. Their compounds, such as common salt, are among the most familiar chemicals.
The family in numbers
Melting points in this family run from 26.9 °C (francium) to 180.5 °C (lithium). Among its solid members, the least dense is lithium (0.53 g/cm³) and the densest is caesium (1.93 g/cm³). The most recently discovered member is francium (1939).
How the family changes down the column
The alkali metals show one of the clearest trends in the whole table. Going down the column the atoms get bigger, the single outer electron sits farther from the nucleus and is lost more easily, and so the metals become more reactive. Dropped in water, lithium fizzes steadily, sodium melts into a silvery ball that skates across the surface, and potassium catches fire at once. Rubidium and caesium react explosively. The metals also get softer and melt at lower temperatures as you go down.
Flame colours
Each alkali metal gives its own colour to a flame, which is an easy laboratory test: lithium gives crimson red, sodium a strong yellow and potassium lilac. The yellow of a sodium street lamp and the yellow flash when salty water boils over on a gas stove are the same light.
Where you meet them
You never meet the pure metals outside a laboratory, but their compounds are everywhere. Sodium chloride is table salt, sodium hydrogencarbonate is baking soda and sodium hydroxide is caustic soda. Potassium compounds are in fertiliser, and plants and animals need potassium to live. Lithium is the heart of the rechargeable batteries in phones and electric vehicles. Hydrogen sits above lithium in most tables because it too has one outer electron, but it is a gas and a nonmetal, not an alkali metal.
All alkali metals compared
| No. | Symbol | Name | Atomic mass | State | Melts at | Boils at | Density g/cm³ | Discovered |
|---|---|---|---|---|---|---|---|---|
| 3 | Li | Lithium | 7.0 | Solid | 180.5 °C | 1,342 °C | 0.53 | 1817 |
| 11 | Na | Sodium | 22.9897693 | Solid | 97.8 °C | 882.9 °C | 0.97 | 1807 |
| 19 | K | Potassium | 39.0983 | Solid | 63.4 °C | 758.9 °C | 0.89 | 1807 |
| 37 | Rb | Rubidium | 85.468 | Solid | 39.3 °C | 687.9 °C | 1.53 | 1861 |
| 55 | Cs | Caesium | 132.9054520 | Solid | 28.4 °C | 670.9 °C | 1.93 | 1860 |
| 87 | Fr | Francium | 223.01973 | Solid | 26.9 °C | no value | no value | 1939 |
Other families: alkaline earth metals, transition metals, post-transition metals, metalloids, nonmetals, halogens, noble gases, lanthanides, actinides.