Group 1 — The Alkali Metals

This lesson covers the Group 1 elements (the alkali metals), as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 6: Groups in the Periodic Table. You need to know the properties of the alkali metals, their reactions with water, and how to explain the trend in reactivity down the group using electron configuration.

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The Group 1 Elements

Group 1 is found on the far left of the periodic table. The elements in Group 1 are called the alkali metals because they react with water to form alkaline solutions (metal hydroxides).

Element	Symbol	Atomic Number	Electron Configuration	Melting Point (°C)	Density (g/cm³)
Lithium	Li	3	2, 1	181	0.53
Sodium	Na	11	2, 8, 1	98	0.97
Potassium	K	19	2, 8, 8, 1	63	0.86
Rubidium	Rb	37	2, 8, 18, 8, 1	39	1.53
Caesium	Cs	55	2, 8, 18, 18, 8, 1	28	1.87

Exam Tip: You only need to know the properties and reactions of lithium, sodium and potassium in detail for the Edexcel GCSE exam, but you should be able to predict the properties of rubidium and caesium by extrapolating the trends.

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Physical Properties of Group 1 Metals

The alkali metals share several distinctive physical properties:

• Soft — they can be cut with a knife, revealing a shiny surface that quickly tarnishes in air.
• Low density — lithium, sodium and potassium are less dense than water (lithium is the least dense metal).
• Low melting and boiling points — compared with most other metals. Melting points decrease down the group.
• Shiny when freshly cut — but rapidly oxidise in air to form a dull oxide layer.
• Good conductors of heat and electricity (as with all metals).

Trends in Physical Properties

As you go down Group 1:

Property	Trend	Explanation
Melting point	Decreases	Metallic bonds become weaker as the atoms get larger and the delocalised electrons are further from the positive nuclei
Boiling point	Decreases	Same reason as melting point
Density	Generally increases	Atoms become heavier (greater atomic mass) faster than they increase in volume
Atomic radius	Increases	Each element has one more electron shell than the one above
Hardness	Decreases	Weaker metallic bonding makes the metal softer

Exam Tip: When asked to explain a trend in physical properties, always link your answer to the structure and bonding. For melting point, refer to the strength of the metallic bonds and the distance of delocalised electrons from the nucleus.

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Reactions of Group 1 Metals with Water

All Group 1 metals react vigorously with cold water. The general word equation is:

alkali metal + water → metal hydroxide + hydrogen

The general symbol equation is:

2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)

where M represents any Group 1 metal.

Specific Reactions

Lithium with water:

Word equation: lithium + water → lithium hydroxide + hydrogen

Symbol equation: 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)

Observations:

• Lithium floats on the water (low density)
• Fizzes steadily as hydrogen gas is produced
• Gradually gets smaller and eventually disappears
• The solution becomes alkaline (turns universal indicator blue/purple)
• No flame is observed

Sodium with water:

Word equation: sodium + water → sodium hydroxide + hydrogen

Symbol equation: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Observations:

• Sodium floats on the water
• Melts into a shiny silver ball (low melting point, exothermic reaction)
• Moves rapidly across the surface of the water, fizzing vigorously
• May ignite with a yellow/orange flame if it gets trapped at the edge
• The solution becomes strongly alkaline
• The sodium gets smaller and eventually disappears

Potassium with water:

Word equation: potassium + water → potassium hydroxide + hydrogen

Symbol equation: 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

Observations:

• Potassium floats on the water
• Immediately catches fire, burning with a lilac/purple flame
• Moves rapidly across the surface
• Fizzes violently — may spit and crackle
• The solution becomes strongly alkaline
• Reacts much more vigorously than sodium

Exam Tip: In the exam, you must describe what you would see (observations), not just the products. Saying "hydrogen is produced" is not an observation — saying "fizzing/bubbling is observed" is. Saying "potassium burns with a lilac flame" is a good observation.

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Trend in Reactivity

Reactivity increases as you go down Group 1. Potassium is more reactive than sodium, which is more reactive than lithium.

Explaining the Trend

All Group 1 metals have one electron in their outer shell. When they react, they need to lose this one outer electron to form a positive ion (M⁺) with a stable noble gas electron configuration.

As you go down the group:

1. The atoms get larger (more electron shells).
2. The outer electron is further from the nucleus.
3. There is more shielding from inner electron shells.
4. The attraction between the nucleus and the outer electron is weaker.
5. The outer electron is lost more easily.
6. Therefore the metal is more reactive.

Metal	Shells	Distance of Outer Electron from Nucleus	Ease of Losing Outer Electron	Reactivity
Li	2	Closest	Hardest	Least reactive
Na	3	Further	Easier	More reactive
K	4	Even further	Even easier	Most reactive (of the three)

Exam Tip: When explaining the trend in reactivity for Group 1, you MUST mention: (1) the outer electron is further from the nucleus, (2) there is increased shielding, and (3) the attraction between the nucleus and the outer electron is weaker — so the electron is lost more easily. All three points are needed for full marks.

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Storage and Safety

Because of their high reactivity, Group 1 metals must be stored carefully:

• Stored under oil to prevent them reacting with oxygen and moisture in the air.
• Handled with forceps — never touched with bare hands.
• Cut with a knife inside a fume cupboard.
• Only small pieces are used in experiments due to the vigorous reactions.

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Predicting Reactions of Rubidium and Caesium

Using the trend in reactivity, we can predict:

• Rubidium would react even more vigorously with water than potassium — likely to explode.
• Caesium would react extremely violently with water — an explosive reaction.
• Both would float (rubidium may just about sink as its density is 1.53 g/cm³).
• Both would produce a hydroxide and hydrogen gas.

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Flame Colours

Each Group 1 metal produces a characteristic flame colour when heated:

Metal	Flame Colour
Lithium	Crimson red
Sodium	Yellow/orange
Potassium	Lilac/purple

These flame colours are used in flame tests to identify the metal ions in compounds.

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Summary

• Group 1 metals are called the alkali metals because they form alkaline hydroxides when they react with water.
• They have low melting points, low densities and are soft.
• They react with water to form a metal hydroxide and hydrogen gas.
• Reactivity increases down the group because the outer electron is further from the nucleus, there is more shielding, and the electron is lost more easily.
• The metals are stored under oil due to their high reactivity.
• Each element has one electron in its outer shell, which it loses to form a 1+ ion.

Exam Tip: A 6-mark question on Group 1 trends is very common. Structure your answer: state the trend, describe the observations for two or three metals, then explain using electron configuration (number of shells, distance from nucleus, shielding, ease of electron loss).

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Worked Examples

Worked Example 1 — Predicting the Reaction of Caesium with Water

Question: Predict and describe the reaction between caesium and water. Write a balanced symbol equation and explain why caesium reacts more vigorously than potassium.

Answer:

Caesium sits below potassium in Group 1, so it will react even more vigorously than potassium. We predict an explosive reaction: a loud bang, spitting droplets of molten metal, and a bright lilac-tinged flame. The metal will melt instantly because of the exothermic nature of the reaction and its low melting point (28 °C).

Word equation: caesium + water → caesium hydroxide + hydrogen

Balanced symbol equation: 2Cs(s) + 2H₂O(l) → 2CsOH(aq) + H₂(g)

The solution would become strongly alkaline (pH 13–14), turning universal indicator dark blue or purple. A characteristic pop would be heard from any hydrogen that ignites.

Explanation of increased reactivity: Caesium has six occupied electron shells (compared with four in potassium). The single outer electron is therefore much further from the nucleus. Shielding by the inner 54 electrons is much greater. The electrostatic attraction between the caesium nucleus and its outer electron is very weak, so the outer electron is lost extremely easily to form Cs⁺. More reactive metal = faster, more exothermic reaction.

Worked Example 2 — Balancing an Equation for Lithium Oxide

Question: Write a balanced symbol equation for the reaction of lithium with oxygen.

Answer:

Word equation: lithium + oxygen → lithium oxide

Unbalanced: Li(s) + O₂(g) → Li₂O(s)

Balance oxygens by placing 2 in front of Li₂O: Li(s) + O₂(g) → 2Li₂O(s)

Now there are 4 Li on the right, so place 4 in front of Li on the left:

4Li(s) + O₂(g) → 2Li₂O(s)

Check: 4 Li on each side, 2 O on each side. Balanced.

Worked Example 3 — Flame-Test Identification

Question: An unknown white powder is thought to contain either lithium chloride or potassium chloride. Describe how you would carry out a flame test and explain how the result would identify the compound.

Answer:

1. Dip a clean nichrome or platinum wire loop in dilute hydrochloric acid to clean off any contaminants.
2. Heat the loop in a blue roaring Bunsen flame until it burns colourless.
3. Dip the clean loop into the powder so a small amount sticks.
4. Hold the loop in the edge of the blue flame.
5. Observe the colour.

A crimson red flame indicates lithium ions (Li⁺). A lilac/purple flame indicates potassium ions (K⁺). Because each Group 1 ion has a characteristic flame colour, this test gives an unambiguous identification. Flame tests work because heated metal ions emit visible light at characteristic wavelengths when their electrons return to lower energy levels.

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Comparison Table — Groups 1, 7 and 0 at a Glance

Feature	Group 1	Group 7	Group 0
Outer electrons	1	7	Full (2 or 8)
Metal / non-metal	Metal	Non-metal	Non-metal
Reactivity trend	Increases down group	Decreases down group	Virtually unreactive
Electron transfer in reactions	Loses 1 e⁻ → M⁺	Gains 1 e⁻ → X⁻	None
Typical compound	MOH (alkaline)	MX (ionic halide)	None common
State at room temp	Solid	Gas, liquid, solid	Gas

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Common Mistakes and How to Avoid Them

Common Mistake 1: Writing "M + H₂O → MOH + H" (forgetting hydrogen is diatomic). Always write H₂ as the hydrogen product.

Common Mistake 2: Forgetting to balance equations. 2Na + 2H₂O → 2NaOH + H₂ is balanced; Na + H₂O → NaOH + H₂ is not (only 1 H on left, 3 on right).

Common Mistake 3: Describing the reaction without observations. Saying "hydrogen is produced" is not an observation — you cannot see hydrogen. Instead say "fizzing/effervescence is observed".

Common Mistake 4: Explaining reactivity by saying "potassium has more electrons". This is too vague — you must mention (i) more shells, (ii) outer electron further from nucleus, (iii) increased shielding, (iv) weaker attraction, (v) electron lost more easily.

Common Mistake 5: Confusing flame colours. Remember: Lithium = Crimson; Sodium = Yellow; K (potassium) = Lilac. A helpful mnemonic is "Little Susie K — Crimson Yellow Lilac".

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Reactivity Flow Diagram

flowchart TD
    A["Group 1 atom M<br/>with 1 outer electron"] --> B{"Reaction with H₂O"}
    B --> C["M loses 1 electron"]
    C --> D["M⁺ ion formed<br/>(full outer shell of M-1)"]
    D --> E["2M + 2H₂O → 2MOH + H₂"]
    E --> F["MOH dissolves → alkaline solution<br/>(pH 13–14)"]

    G["Going down group:"] --> H["More shells → larger atom"]
    H --> I["Outer e⁻ further from nucleus"]
    I --> J["More shielding"]
    J --> K["Weaker attraction"]
    K --> L["Electron lost more easily<br/>→ MORE REACTIVE"]

    style A fill:#3498db,color:#fff
    style F fill:#27ae60,color:#fff
    style L fill:#e74c3c,color:#fff

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Answering at different grade levels

Grade 3–4 answer (basic recall): "Potassium is more reactive than sodium. It fizzes more when you put it in water and it sometimes burns with a lilac flame colour. Group 1 reactivity increases down the group."

Grade 5–6 answer (using precise terms): "Potassium is more reactive than sodium because potassium's outer electron is further from the nucleus. When potassium reacts with water it produces potassium hydroxide and hydrogen gas. The observations include a lilac flame colour, fizzing, the metal melting into a ball, and the solution turning alkaline."

Grade 7–9 answer (full electron-transfer explanation with oxidation state): "Reactivity increases down Group 1 because each descending element has an additional electron shell, so the outer electron is progressively further from the nucleus and experiences greater shielding from inner-shell electrons. The net nuclear attraction on the outer electron is therefore weaker and the electron is lost more readily, producing a 1+ cation with a stable noble-gas configuration. In electron-transfer terms, the alkali metal is oxidised (oxidation state 0 → +1) while the water is reduced. Writing the half equation M → M⁺ + e⁻ makes this electron-loss explicit. This explains why caesium reacts explosively with water whereas lithium merely fizzes steadily — the same electron transfer occurs, but the activation barrier to lose the outer electron is much lower in caesium."

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Edexcel alignment: This content is aligned with Edexcel GCSE Chemistry (1CH0) specification Topic 7 Groups in the periodic table — specifically 7.1 Group 1 (physical properties of the alkali metals, reactions with water, reactivity trend explained by electron configuration, storage) and links to 7.3 Group 0 (contrast with unreactive elements) and identification tests (flame tests). Assessed on Paper 1.