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K223 / K225 / 2027
Periodicity and Metals

Lesson 3 of 8

Group 1: lithium, sodium and potassium

Similar products, increasingly vigorous reaction with water.

SEC G2 Science K223 / K225 (2027)

  • I can predict group properties.Syllabus

    Syllabus K223 / K225, 7.1(f). Predict group properties

  • I can compare alkali metals.Syllabus

    Syllabus K223 / K225, 7.2(a). Compare alkali metals

Group 1: lithium, sodium and potassium

Similar products, increasingly vigorous reaction with water.

Make a guess. You are not marked.

A small piece of sodium reacts quickly with cold water. What does the same size piece of potassium do?

  1. It reacts less vigorously, as its atoms are bigger
  2. It does not react, because it has a full outer shell
  3. It reacts more vigorously, losing its electron more easily
Show the answer

It reacts more vigorously, losing its electron more easily

Potassium is further down Group 1. Its outer electron is further from the nucleus and lost more easily, so it reacts faster.

Lithium, sodium and potassium are relatively soft, low-density metals. Their melting points decrease down this sequence, while their reactions with cold water become more vigorous. Low density is a shared property; do not assume density follows a perfectly steady trend.

Reaction with cold water
MetalTypical observationProducts
LithiumFloats and fizzes steadilyLithium hydroxide and hydrogen
SodiumFloats, moves and may melt into a ball as heat is releasedSodium hydroxide and hydrogen
PotassiumReacts very vigorously; hydrogen may ignitePotassium hydroxide and hydrogen

For sodium: 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g). The resulting solution is alkaline because it contains hydroxide ions. The shared outer electron explains the common product pattern. Use observed trends to predict an unfamiliar group-1 element, while treating predicted observations as predictions rather than measured facts.

Check your understanding

Using the lithium to potassium trend, how should the Group 1 element below potassium compare with potassium?

  1. It should have a higher melting point, because it has more protons and a bigger atom.
  2. It should react more vigorously with water and have a lower melting point.
  3. It should react less vigorously, because its outer electron is further from the nucleus.
Show the answer

It should react more vigorously with water and have a lower melting point.

Down Group 1, reactivity rises and melting point falls, so the next element continues both trends. It still forms a hydroxide and hydrogen. A trend cannot give exact rates or temperatures.

Check your understanding

Rubidium is below potassium in Group 1. Which prediction is most likely to be correct?

  1. It reacts with water to form an acidic solution of rubidium ions and hydrogen
  2. It does not react with water, because its outer electron is far from the nucleus
  3. It has a higher melting point than potassium, because its atoms are heavier
  4. It reacts more vigorously with cold water than potassium does, forming RbOH and hydrogen
Show the answer

It reacts more vigorously with cold water than potassium does, forming RbOH and hydrogen

Reactivity with water increases down Group 1, so rubidium reacts even more vigorously: 2Rb + 2H2O -> 2RbOH + H2.

Exam-style question

The melting points of three Group 1 metals are: lithium 181 °C, sodium 98 °C, potassium 63 °C. Rubidium is the next element down the group.
(a) Predict the melting point of rubidium. [1]
(b) Predict how rubidium reacts with cold water, and name both products. [2]
(c) Explain why lithium, sodium and potassium react with water in a similar way. [1] [4]

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