Group 1 Elements: The Alkali Metals
Learn Group 1 alkali-metal properties, melting-point and reactivity trends, water-reaction observations, balanced equations and exam explanations.
On this page
Lithium, sodium and potassium share a reaction pattern: each reacts with water to form an alkaline solution and hydrogen gas. The products stay the same, but the reaction becomes more vigorous down Group 1. This lesson connects those observations with electron arrangements and predictions.
Compare Group 1 using the interactive periodic table.
What makes an alkali metal?
The Group 1 metals are called alkali metals because their reactions with water produce soluble metal hydroxides. These solutions contain hydroxide ions, OH⁻, and are alkaline.
Hydrogen is placed in Group 1 because its atom also has one outer electron, but hydrogen is a non-metal, not an alkali metal. Here we compare lithium, sodium and potassium.
One outer electron, one positive charge
The neutral atoms have electron arrangements Li: 2,1, Na: 2,8,1 and K: 2,8,8,1. Each metal atom can lose its one outer electron to form an ion with charge + 1:
Na → Na⁺ + e⁻
Losing a negative electron leaves one more proton than electron. The same pattern explains why these metals form similar compounds, such as LiOH, NaOH and KOH.
Physical properties and melting points
| Property | Lithium, sodium and potassium |
|---|---|
| Appearance | Silvery and shiny when freshly cut; tarnish quickly in air. |
| Softness | Soft enough to cut with a knife. |
| Melting point | Relatively low for metals; decreases from lithium to potassium. |
| Density | All three are less dense than water and float. Potassium is less dense than sodium, so density does not increase steadily through these three. |
| Conductivity | Conduct heat and electricity. |
Data table
| Metal, in order down Group 1 | Melting point |
|---|---|
| Lithium | 180.5 |
| Sodium | 97.8 |
| Potassium | 63.5 |
Values are rounded from the Royal Society of Chemistry’s data for lithium, sodium and potassium. You need the decreasing trend, rather than these numerical values, for recall questions.
Use a trend to make a prediction
An unfamiliar Group 1 metal below potassium is expected to have a lower melting point and react more vigorously with water. State the observed trend before extending it. The three bars do not establish a fixed decrease in temperature, so they cannot supply an exact melting point for the next metal.
Reactions with water
For a Group 1 metal represented by M:
2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)
For example, sodium forms sodium hydroxide and hydrogen:
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
| Metal | Typical observations with a small piece in water |
|---|---|
| Lithium | Floats, fizzes gently and moves slowly. |
| Sodium | Floats, melts into a silvery ball, moves rapidly and fizzes vigorously. |
| Potassium | Floats and reacts very vigorously; may ignite with a lilac flame. |
The bubbles are hydrogen gas. A lighted splint gives a squeaky pop with hydrogen. The solution becomes alkaline because the dissolved hydroxide provides OH⁻ ions.
Under comparable conditions, increasing reaction vigour is evidence that reactivity increases: lithium < sodium < potassium. Down the group, the outer electron is farther from the nucleus and shielded by more inner electron shells. It is lost more easily, helping to explain the increasing reactivity.
Lithium, sodium and potassium are stored under oil to limit contact with air and moisture. Water reactions require very small pieces and teacher-controlled conditions. Potassium is unsuitable for an unsupervised student experiment.
Keep observations and explanations separate
- Observation: “Sodium fizzes vigorously.” Conclusion: “It is more reactive than lithium under the comparison conditions.”
- Bubbles show that a gas is formed; the squeaky-pop test identifies hydrogen.
- Group 1 metals form hydroxides, not oxides, in these water reactions.
- Equal masses of different metals need not have the same exposed surface area. Piece size and experimental conditions can affect the apparent reaction vigour.
Build an explanation from the evidence
Name the metals, compare the observations and state the supported trend. For a water-reaction equation, identify hydroxide + hydrogen, then balance atoms and add state symbols. A flame colour is an observation that may be supplied; it does not replace the comparison of reaction vigour.
Worked examples
Modelled example 1
Predict products (water reaction)
Problem
Write the balanced equation, including state symbols, for potassium reacting with water.
Study the worked solution
Predict the products
Method
Use the Group 1 water-reaction pattern.Reason
A Group 1 metal reacts with water to form its soluble hydroxide and hydrogen.Working
K + H₂O → KOH + H₂.Balance atoms
Method
Use coefficients of 2 for potassium, water and potassium hydroxide.Reason
This gives two K, four H and two O atoms on each side.Working
2K + 2H₂O → 2KOH + H₂.Add physical states
Method
Label the metal, water, dissolved hydroxide and gas.Reason
Potassium is solid, water is liquid, potassium hydroxide is aqueous and hydrogen is gaseous under the stated conditions.Working
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
Guided practice 2
Trend from evidence
Problem
Lithium fizzes gently with water, sodium reacts vigorously and potassium reacts very vigorously. State the trend supported by these observations.
Turn observations into an ordered claim
Hints
Hint 1: order the evidence
Arrange gentle, vigorous and very vigorous from least to most.
Hint 2: connect vigour to reactivity
Under comparable conditions, a more vigorous reaction supports greater reactivity.
View solution step by step
Order the observations
Method
Compare reaction vigour under the stated conditions.Reason
Vigour provides the observable evidence for relative reactivity.Working
Lithium < sodium < potassium in reaction vigour.State the group trend
Method
Map the order onto positions down Group 1.Reason
Lithium, sodium and potassium occur successively lower in the group.Working
Reactivity increases down Group 1.
Common misconception 3
Link outer electrons to ion charge
Learner response
Track the negative electron
View solution step by step
Locate the direction error
Method
Replace “gains” with “loses.”Reason
Sodium has one outer electron and reaches a stable outer-shell arrangement by removing it.Working
Na → Na⁺ + e⁻.Explain the sign
Method
Compare proton and electron charges after the loss.Reason
Losing one negative charge leaves one net positive charge.Working
Sodium forms Na⁺.
Examiner practice 4
Identify the gas
Examination question
A colourless gas is produced when sodium reacts with water. State the test and positive result used to identify the gas. [2 marks]
Name the procedure and observation
View solution step by step
State the test
1 markMethod
Bring a lighted splint to the gas.Reason
This is the standard test for hydrogen.Working
Test: lighted splint.State the positive result
1 markMethod
Listen for a squeaky pop.Reason
Hydrogen ignites rapidly in the presence of oxygen.Working
Positive result: squeaky pop → hydrogen.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the test and positive observation separately.
Challenge 5
Predict a melting-point trend
Trend extrapolation
The melting points of lithium, sodium and potassium decrease in that order. Predict how the next Group 1 element’s melting point is likely to compare with potassium, and state the limit of the prediction.
Extend direction without inventing a number
Hints
Hint 1: continue the supplied order
Lithium to sodium to potassium is both downward in the group and downward in melting point.
Hint 2: separate direction from magnitude
A trend direction does not supply the size of the next change.
View solution step by step
State the supplied trend
Method
Identify decreasing melting point down Group 1.Reason
The three named elements establish a qualitative direction.
Working
Tₘ(Li) > Tₘ(Na) > Tₘ(K).
Extend and bound the claim
Method
Predict a lower melting point for the next element.
Reason
This follows the given trend, but no spacing or equation supports an exact value.
Working
Likely lower than potassium; exact value cannot be claimed from these data.
Try these independently
Mind stretcher 1: Predict an unknown elementExtension
Question: Element X is below potassium in Group 1. Predict how its reaction with water would compare with potassium and explain your prediction.
Show Answer
X should react more vigorously than potassium because the observed reactivity trend increases down Group 1.
Mind stretcher 2: Evaluate a fair comparisonExtension
Question: A student compares a large piece of lithium with a tiny piece of sodium in water and concludes lithium is more reactive. Why is the conclusion unreliable?
Show Answer
The pieces differ in amount and exposed surface area, so their reaction vigour cannot be attributed only to metal identity. A teacher-controlled comparison should use similarly small pieces with comparable exposed surfaces, and the same water temperature and volume. Equal mass alone does not control surface area.
Practise and check
Test physical properties, water-reaction observations, balanced equations, ion formation and trend explanations.
Open the Periodic Table topic checkSyllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
Last reviewed: