Group I properties

O Level / IGCSE Chemistry · The Periodic Table. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Lithium, sodium and potassium are the alkali metals of Group I. Each atom has one outer-shell electron, which it loses easily to form a 1+ ion. This is why they all react in the same way.

They are soft enough to cut with a knife and shiny when freshly cut. For metals, they have low densities and low melting points. They react quickly with oxygen and water vapour in the air, so they are stored under oil.

With cold water they form hydrogen gas and a solution of the metal hydroxide, which is strongly alkaline. Going down the group the melting point decreases, the density generally increases and the reactivity increases.

Rules to remember

Common mistake

Students give the Group I reactivity trend the wrong way round, copying the halogens. In Group I reactivity increases down the group; in Group VII it decreases.

Worked example

Equal-sized pieces of lithium and potassium are added to separate troughs of cold water. How do the observations compare, and what do the two solutions have in common?

  1. Both are Group I metals, so both float, fizz as hydrogen is given off and get smaller until they disappear.
  2. Potassium is lower in the group, so it is more reactive: it reacts much faster, melts and burns with a lilac flame. Lithium only fizzes steadily.
  3. Both form a metal hydroxide: 2Li + 2H2O → 2LiOH + H2 and 2K + 2H2O → 2KOH + H2.

Answer: Potassium reacts far more vigorously than lithium. Both leave a colourless alkaline solution of the metal hydroxide, which turns universal indicator purple.

Practice questions

Try each one, then open the answer.

1. Which statement about the Group I metals lithium, sodium and potassium is correct?

  1. A
    they are liquids at room temperature
  2. B
    they are relatively soft and can be cut with a knife
  3. C
    they form negative ions by gaining one electron
  4. D
    they are hard with high melting points
Show answer

Answer: B. Group I metals are soft enough to cut with a knife, showing a shiny surface that quickly tarnishes. They form 1+ ions.

2. Which trend is shown going down Group I of the Periodic Table?

  1. A
    reactivity decreases as the atoms get bigger
  2. B
    melting point increases as the metallic bonds get stronger
  3. C
    the metals become harder to cut with a knife
  4. D
    density generally increases down the group
Show answer

Answer: D. Going down Group I, density generally increases, melting point decreases and reactivity increases.

3. Caesium is below potassium in Group I. Which prediction about caesium is correct?

  1. A
    it reacts more slowly with cold water than potassium, forming caesium oxide
  2. B
    it forms Cs2+ ions because it has two outer electrons
  3. C
    it reacts violently with cold water, forming caesium hydroxide
  4. D
    it has a higher melting point than potassium and is harder to cut
Show answer

Answer: C. Reactivity increases down Group I, so caesium is more reactive than potassium. Like all Group I metals it forms 1+ ions and a hydroxide with water.

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