Group 1: The Alkali Metals

Reactions with water become more vigorous down the group

• All Group 1 + water reactions give the same products, but the speed and energy of each reaction increase as you go down the group

• Balanced symbol equations:
  • 2 Li(s) + 2 H₂O(l) → 2 LiOH(aq) + H₂(g)
  • 2 Na(s) + 2 H₂O(l) → 2 NaOH(aq) + H₂(g)
  • 2 K(s) + 2 H₂O(l) → 2 KOH(aq) + H₂(g)

Reactions with oxygen become more vigorous down the group

• The alkali metals tarnish in air: a dull coating of metal oxide builds up on a freshly cut surface within seconds
• Tarnishing is faster the further down the group the metal sits — this is why the metals are stored under oil
• The general equation is 4 M(s) + O₂(g) → 2 M₂O(s):
  • 4 Li(s) + O₂(g) → 2 Li₂O(s)
  • 4 Na(s) + O₂(g) → 2 Na₂O(s)
  • 4 K(s) + O₂(g) → 2 K₂O(s)

Physical trends down the group

• Softness increases: lithium can be cut with a knife with some effort, caesium with almost none
• Density rises overall down the group (Li 0.53 g cm⁻³, Na 0.97, K 0.86, Rb 1.53, Cs 1.93)
• Melting point falls down the group (Li 181 °C, Na 98 °C, K 63 °C, Rb 39 °C, Cs 29 °C)

Predicting properties of rubidium, caesium and francium

• Reactivity continues to increase, so the heavier alkali metals react with water explosively
• Rubidium should be a soft, silvery metal that is denser than potassium and melts below 63 °C
• Caesium is even softer, denser and lower-melting than rubidium
• Francium is radioactive and short-lived, but is predicted to be the most reactive Group 1 metal of all