1.13 Group 1: the alkali metals

The alkali metals

Their atoms all have one electron in the outer shell, so they react in similar ways.

Physical properties:

• soft: they can be cut with a knife
• shiny when freshly cut, but they quickly tarnish (go dull) as they react with oxygen in the air
• low density: lithium, sodium and potassium float on water
• low melting points, which decrease down the group

They are stored under oil to stop them reacting with oxygen and water vapour in the air.

Reactions with water

metal + water → metal hydroxide + hydrogen

2Na + 2H₂O → 2NaOH + H₂

The metal hydroxide dissolves to form an alkaline solution (universal indicator turns purple).

Reactions with oxygen and chlorine

• With oxygen they form metal oxides: 4Na + O₂ → 2Na₂O
• With chlorine they form metal chlorides, which are white solids: 2Na + Cl₂ → 2NaCl

The products are ionic compounds containing 1+ metal ions, such as \mathrm{Na^+}. They are white solids that dissolve to give colourless solutions.

Why reactivity increases down the group

In every reaction, the atom loses its one outer electron to form a 1+ ion. Going down the group:

1. The atoms have more shells, so the outer electron is further from the nucleus.
2. There is more shielding by the inner electrons.
3. So the attraction between the nucleus and the outer electron is weaker.
4. The outer electron is lost more easily, so the metal is more reactive.

You can use this to predict that rubidium and caesium react even more violently, exploding in water.