2.1 Group 1 (Alkali Metals) Notes

Edexcel IGCSE Chemistry: Revision notes

Key facts

  • Alkali metal + water gives metal hydroxide + hydrogen, leaving an alkaline solution.
  • Reactions with water and air become more vigorous down the group: Li < Na < K.
  • Rubidium and caesium can be predicted to be even more reactive, with lower melting points.
  • All have one outer electron, lost to form a 1+ ion.
  • Reactivity increases down the group because the outer electron is further from the nucleus and more shielded.
1234567012341H2He3Li4Be5B6C7N8O9F10Ne11Na12Mg13Al14Si15P16S17Cl18Ar19K20Ca21Sc22Ti23V24Cr25Mn26Fe27Co28Ni29Cu30Zn31Ga32Ge33As34Se35Br36Kr
Group 1: lithium, sodium and potassium, with rubidium and caesium below.

Reaction with water

All three give a metal hydroxide and hydrogen, which is evidence they belong to one family.

Lithium, sodium and potassium all float on water, move around, fizz as hydrogen is released, and gradually dissolve to leave a colourless alkaline solution (universal indicator turns purple).

2 Na(s)+2 HX2O(l)→2 NaOH(aq)+HX2(g)\ce{2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)}

  • Alkali metal + watermetal hydroxide + hydrogen

Which two products form when an alkali metal reacts with water?

Trend in reactivity

Reactions get more vigorous and tarnishing faster going down the group.

The reactions are similar, but their vigour increases down the group. This is evidence of a trend of increasing reactivity down Group 1.

Lithium

With water:
Fizzes gently, moves slowly
With air:
Tarnishes slowly

Sodium

With water:
Fizzes more vigorously, may melt into a ball
With air:
Tarnishes faster

Potassium

With water:
Reacts violently, may ignite with a lilac flame
With air:
Tarnishes very quickly; stored under oil

Which alkali metal tarnishes fastest in air?

Predicting properties

Rubidium and caesium should be more reactive than potassium, with lower melting points.

Because reactivity increases down the group, rubidium (Rb) and caesium (Cs) will react even more vigorously with water and air than potassium, have lower melting points, and still form a hydroxide and hydrogen with water. Caesium is expected to react explosively with water at room temperature.

050100150LiNaKRbCsAlkali metalMelting point (°C)
Melting points of the alkali metals (°C)

Predict how rubidium compares with potassium.

Why reactivity increases

Down the group the outer electron is further away and more shielded, so it is lost more easily.

Alkali metal atoms all have one outer electron, which they lose to form a 1+ ion. Going down the group, atoms have more shells, so the outer electron is further from the nucleus and shielded by more inner shells. It is less strongly attracted and easier to lose.

Li3p 4n

Lithium

2,1

Na11p 12n

Sodium

2,8,1

K19p 20n

Potassium

2,8,8,1

One outer electron each; further from the nucleus down the group.
  1. 1

    More shells

    atoms get larger going down

  2. 2

    Further and shielded

    the outer electron is further from the nucleus with more shielding

  3. 3

    Weaker attraction

    the nucleus holds it less strongly

  4. 4

    Easier to lose

    so the metal is more reactive

Why reactivity increases down Group 1

Why is potassium more reactive than sodium?

Try an exam question

Explain why potassium is more reactive than lithium.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.