The alkali metals
Group 1 contains the alkali metals: lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and caesium (Cs). Each atom has one valence electron, which it loses easily to form a +1 ion with a stable electron arrangement.
Physical properties
They are soft, silvery metals with low densities and low melting points — sodium and potassium can be cut with a knife. They are stored under oil because they react rapidly with air and moisture.
Reaction with water
Alkali metals react vigorously with water to form an alkaline hydroxide and hydrogen gas.
Key idea
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
The solution formed turns red litmus blue because it is alkaline.
Trend in reactivity
Reactivity increases down the group. As atoms get larger, the single valence electron is further from the nucleus and more weakly held, so it is lost more easily. Lithium fizzes gently, sodium melts into a ball and darts about, and potassium ignites with a lilac flame.
Worked example
Predict the products when potassium reacts with water.
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Potassium hydroxide (an alkali) and hydrogen gas are formed; the reaction is faster than sodium's because K is lower in the group.
Remember
- One valence electron → +1 ions.
- React with water to give hydroxide + hydrogen.
- Reactivity increases down the group.