Lithium, sodium and potassium are metals you can cut with a knife and store under oil. Drop them in water and they fizz, float and, further down the group, burst into flame. Their trends let you predict what rubidium and caesium will do.
The Group I alkali metals are soft, have relatively low melting points and low densities, and are good conductors. They react with cold water to form an alkaline hydroxide and hydrogen: 2Na + 2H2O → 2NaOH + H2. Down the group, the melting point decreases, the density generally increases and the reactivity increases. Their outer electron is further from the nucleus and more easily lost.
Fig. 1: melting points of the Group I metals
Section 01
Reactions with water.
Each alkali metal reacts with water in the same way, but the violence increases down the group.
Metal
Observations with cold water
lithium
floats, fizzes steadily, slowly disappears
sodium
floats, melts into a silvery ball, fizzes rapidly, moves around on the surface
potassium
floats, melts, fizzes violently, the hydrogen ignites with a lilac flame
Products: a metal hydroxide (alkaline solution, turns universal indicator purple) and hydrogen.
Storage: under oil, to stop them reacting with air and moisture.
Stage 1 · Learn
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What are the products when sodium reacts with water?
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How does the reactivity of the Group I metals change down the group?
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Why are alkali metals stored under oil?
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How does the melting point change down Group I?
Section 02
Trends and predictions.
Step down Group I in the simulator and watch the atoms get bigger. The outer electron is further from the nucleus and more shielded, so it is lost more easily.
Melting point: decreases down the group (lithium 181 °C, sodium 98 °C, potassium 63 °C).
Density: generally increases down the group.
Reactivity: increases down the group.
Predicting rubidium: even lower melting point, reacts explosively with water to form rubidium hydroxide and hydrogen.
Examiner trap
Reactivity increases down Group I but decreases down Group VII. Do not carry the Group I trend over to the halogens. And the solution formed with water is alkaline because of the hydroxide, not because of the hydrogen gas.
Stage 2 · Exam
Exam-style questions
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A small piece of potassium is added to water containing universal indicator. What is seen?
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Which is the balanced equation for the reaction of lithium with water?
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Caesium is below rubidium in Group I. Which prediction about caesium is most likely correct?
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Why is potassium more reactive than sodium?
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Group I, mastered.
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