Group I metals: trends and careful predictions
Recall lithium, sodium and potassium, and distinguish the general trend from supplied exceptions.
Section C8.2 · Group I properties
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Explain one outer electron and +1 ion formation.
- Distinguish melting point, density and chemical reactivity.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What common simple-ion charge belongs to these Group I metals?
What you will learn
- Describe lithium, sodium and potassium as relatively soft alkali metals with the required general trends.
- Predict another Group I element's properties from its position and supplied evidence.
Keep the three syllabus properties separate
Lithium, sodium and potassium are relatively soft Group I alkali metals. Down the group, the syllabus general trends are decreasing melting point, increasing density and increasing reactivity. For the three named metals, reactivity increases from lithium through sodium to potassium, while their melting points decrease. Softness is a mechanical property and is not the same as being liquid at room temperature.
The density trend is general rather than exception-free: potassium is less dense than sodium. Therefore a question supplying measured density values must be answered from those values, not by overwriting the exception with a memorised trend. Density compares mass per volume and does not simply equal the relative atomic mass. All three named metals are solids at room temperature despite their relatively low melting points compared with many other metals.
Use position and the information supplied
A lower Group I position supports a prediction of greater reactivity and a lower melting point in the stated trend comparison. Give the direction rather than inventing an exact temperature or rate. The common +1 ion formation follows the single outer electron, but it does not mean lithium, sodium and potassium react at identical speeds or have equal densities.
Their reactions with water produce an alkaline hydroxide solution and hydrogen; a balanced sodium example is 2Na+2H₂O→2NaOH+H₂. This supports the alkali-metal context, but the lesson's required trend predictions remain limited to melting point, density and reactivity. Highly reactive metal demonstrations require specialist teacher supervision; do not use the equation as an instruction to handle or mix these metals yourself.
Sources for this lesson
Pause and explain
Which statement keeps the density trend accurate?
Worked example
Compare sodium and potassium for reactivity and melting point, then assess a claim that potassium must be denser simply because it is lower in Group I.
Show the worked solution
- Potassium is more reactive than sodium in the named Group I comparison.
- Potassium has a lower melting point in the stated trend.
- The general density increase has an exception here: potassium is less dense than sodium, so lower position is not proof for this pair.
Answer Potassium is more reactive with lower melting point, but it is less dense than sodium.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the required Group I general trends in melting point, density and reactivity down the group.
Give me a hint
Name the property before its direction.
Compare my reasoning
- Melting point generally decreases.
- Density generally increases, with the sodium/potassium exception retained.
- Reactivity increases down the stated Group I comparison.
Melting point decreases; density generally increases; reactivity increases.
Look for these in your work
- All three property directions are stated.
- General density language is preserved.
Predict reactivity and melting point for a Group I element below potassium, if the stated trends continue. Explain why no exact numerical value is given.
Give me a hint
Give a direction based on position, not a fabricated rate or temperature.
Compare my reasoning
- Greater reactivity is predicted from the down-group reactivity trend.
- A lower melting point is predicted from the melting-point trend.
- Exact values require data; group position alone does not supply a numerical rate or temperature.
Predicted more reactive and lower melting point, without exact numerical values.
Look for these in your work
- Both directions follow the stated trends.
- The limit of the prediction is clear.
Supplied densities for sodium and potassium are 0.97 and 0.89 g/cm³. Compare the mass of equal 10 cm³ samples and explain the general-trend limitation.
Give me a hint
Use mass=density×volume for the given pair.
Compare my reasoning
- Sodium mass is 0.97×10=9.7 g.
- Potassium mass is 0.89×10=8.9 g.
- Potassium's sample has lower mass at equal volume, illustrating the exception rather than disproving every overall Group I tendency.
9.7 g sodium versus 8.9 g potassium; potassium is less dense for this supplied pair.
Look for these in your work
- Both masses use the same volume.
- The exception is reported without altering the data.
Common mistakes
- Treating the general density trend as strictly increasing for every pair.
- Calling a soft metal necessarily liquid at room temperature.
- Using atomic mass alone as density.
- Inventing an exact property value from group position.
What can you explain now?
Order lithium, sodium and potassium by increasing reactivity.
Compare with the explanation
Lithium, sodium, potassium.
The required named Group I comparison has increasing reactivity down the group.
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Make it stick
Recall the three named metals and property directions. Explain the density exception and predict a further element only to the precision supported by the question.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.