Alkali metal: definition in chemistry
Alkali metal: Any element in group 1 of the periodic table except hydrogen: lithium, sodium, potassium, rubidium, cesium and francium.
Alkali metals have a single valence electron, which they lose easily to form +1 ions, making them the most reactive metals. They are soft enough to cut with a knife and react with water to form a strongly basic hydroxide and hydrogen gas, for example 2Na + 2H₂O → 2NaOH + H₂. Because they also react with oxygen and moisture in air, they are stored under oil, and their reactivity increases down the group.
Examples
- Lithium — used in rechargeable batteries
- Sodium — part of table salt, NaCl
- Potassium — reacts with water fast enough to ignite the hydrogen
- Lithium, sodium and potassium are all less dense than water
On the periodic table
The alkali metals make up group 1, the first column on the left of the table; hydrogen sits at the top of that column but is a nonmetal.