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Group 1: reactions and trends

Key idea: Group 1 metals form hydroxides and hydrogen with water. Reactivity increases down the group because the outer electron is farther from the nucleus and more easily lost.

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Lithium, sodium and potassium form similar products with water, but react with different vigour. Use their observations and data to identify the Group 1 pattern, explain it through electron loss, and predict a member you have not met.

You need the Periodic Table position rules: occupied shells give the period, and outer electrons help identify the group.

Group 1: the alkali metals

Lithium, sodium and potassium are the Group 1 metals in this course. Start with the evidence from supervised teacher demonstrations. In each water test, a small piece of metal of similar size was added to water under the same conditions.

ElementElectron arrangementMelting point (°C)Density (g/cm³)With cold water
lithium2,11810.53floats; fizzes steadily; slowly gets smaller
sodium2,8,1980.97floats; melts into a silvery ball; fizzes rapidly and moves about
potassium2,8,8,163.50.89floats; melts; fizzes violently; may ignite; a lilac flame is seen

What the data show:

  • All three are soft enough to cut with a knife, and all have a low density: less than water (1.00 g/cm³), so they float. The densities do not change in one direction, so there is no density trend to learn.
  • Melting point decreases down the group: 181 °C, then 98 °C, then 63.5 °C.
  • Reaction with water becomes more vigorous down the group.

Each reaction gives hydrogen and a metal hydroxide solution. The lilac colour seen when potassium burns comes from potassium species in the flame; hydrogen alone does not give a lilac flame. Universal indicator added to the water afterwards turns purple, so the solution is alkaline. That is why Group 1 metals are called the alkali metals.

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Lithium and potassium follow the same equation with Li or K in place of Na.

Compare Group 1 water reactions: lithium, sodium and potassiumAll three metals float and produce an alkaline metal hydroxide solution and hydrogen. Lithium fizzes gently and moves slowly. Sodium melts into a ball, moves rapidly and fizzes vigorously. Potassium reacts very vigorously and may ignite with a lilac flame. The sketches compare typical observations under controlled conditions; depicted sizes and bubbles are not quantitative data.Lithium, LiLeast vigorouswaterFloats; gentle fizzingMoves slowlyMetal hydroxide solution+ hydrogen gasSodium, NaMore vigorouswaterMelts into a ballMoves rapidly; fizzesMetal hydroxide solution+ hydrogen gasPotassium, KMost vigorouswaterVery vigorous fizzingMay ignite: lilac flameMetal hydroxide solution+ hydrogen gas
In teacher-controlled comparisons with similar small pieces and comparable conditions, reaction vigour increases from lithium to potassium. These sketches show typical observations, not measurements of metal amounts or reaction rates.

Why reactivity increases. A Group 1 atom reacts by losing its single outer electron to form a 1+ ion. Going down the group, the atom has more occupied shells. The outer electron is farther from the nucleus and more shielded by the inner shells, so it is held less strongly and lost more easily. The more easily the electron is lost, the more reactive the metal.

Guided practice 1

Identify Group 1 metals from observations

About 5 min

Problem

Equal small pieces of three Group 1 metals are added to water. Metal A fizzes steadily. Metal B melts into a ball and fizzes rapidly. With metal C, a lilac flame is seen as the reaction mixture ignites. Match A, B and C to lithium, sodium and potassium.

Use increasing vigour down the group

Metal A
Metal B
Metal C

Hints

Hint 1

Trend

Compare each metal’s observed vigour with the table.

Hint 2

Distinctive signs

A molten ball suggests sodium; a lilac flame suggests potassium.

Show solution step by step

  1. Rank the observations

    Method

    Arrange the reactions from least to most vigorous.

    Reason

    The pieces are similarly sized and the water conditions are the same, so the observations support this comparison.

    Working

    Answer

    A < B < C in vigour.

  2. Match the metals

    Method

    Assign lithium, sodium and potassium in that order.

    Reason

    Reactivity increases down Group 1, and the molten ball and lilac flame agree.

    Working

    Answer

    A = lithium; B = sodium; C = potassium.

Check your understanding 1 Explain the increasing reactivity

Lithium, sodium and potassium have one outer electron each. Explain why reaction with water becomes more vigorous down Group 1. Connect occupied shells, shielding, attraction and electron loss. Work on paper or in the optional notes.

Show answer

Each atom loses one outer electron to form a 1+ ion. Down the group there are more occupied shells; the outer electron is farther from the nucleus and more shielded by inner electrons. It is held less strongly and lost more readily, so reactivity increases. The common electron count explains similar products; easier loss explains the different vigour.

Worked example 2

Predict the properties of rubidium

Problem

Rubidium is the next element below potassium in Group 1. Predict its melting point, its reaction with water and the ion it forms. Say which property the table cannot help you predict.
Show full solution

  1. Extend the melting-point trend

    Method

    Melting point falls down the group.

    Reason

    181 °C, 98 °C, 63.5 °C is a steady decrease.

    Working

    Answer

    Rubidium is expected to melt below 63.5 °C.

  2. Extend the reactivity trend

    Method

    Its outer electron is even farther from the nucleus and more shielded by inner electrons.

    Reason

    That electron is lost even more easily than potassium’s.

    Working

    Answer

    It is expected to react even more violently with water, forming rubidium hydroxide and hydrogen.

  3. Use the group pattern

    Method

    It has one outer electron, like the rest of Group 1.

    Reason

    Losing that electron gives a 1+ ion.

    Working

    Answer

    Rb⁺

  4. State the limit

    Working

    Answer

    These data cannot reliably predict its density: density does not change steadily from lithium to potassium.

Check your understanding 2 Predict caesium

Caesium is below rubidium in Group 1. Predict its ion charge and how its melting point and water reaction compare with rubidium’s. Can the lithium–potassium density data give a reliable prediction of its exact density?

Melting point compared with rubidium
Water reaction compared with rubidium
Can these data reliably predict the exact density?
Show answer

Caesium forms a 1 + ion. Extending the trends predicts a lower melting point and a more vigorous water reaction than rubidium. The supplied density data do not give a reliable exact density.

Before you move on

Group 17 uses a different reaction pattern. The halogens and displacement lesson compares chlorine, bromine and iodine, and explains why their reactivity decreases down the group.

The noble gases and group comparisons lesson explains unreactivity and brings the three group patterns together.

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Syllabus and review details

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