What you'll learn
This revision guide covers the properties and reactions of Group 1 elements (alkali metals) as required by the AQA GCSE Chemistry specification. You'll learn about their physical and chemical properties, how they react with water and oxygen, and why their reactivity increases down the group. Understanding these patterns is essential for Paper 1 questions worth 4-6 marks.
Key terms and definitions
Alkali metals — the elements in Group 1 of the periodic table (lithium, sodium, potassium, rubidium, caesium and francium), which form alkaline solutions when they react with water.
Reactivity — the tendency of a substance to undergo chemical reactions; for Group 1 metals, reactivity increases down the group.
Electron shielding — the blocking effect that inner shell electrons have on the attraction between the nucleus and outer shell electrons.
Alkali — a soluble base that forms hydroxide ions (OH⁻) in aqueous solution, producing a pH greater than 7.
Displacement reaction — a reaction where a more reactive element takes the place of a less reactive element in a compound.
Oxidation — the loss of electrons during a reaction; all Group 1 metals are oxidised when they react, forming +1 ions.
Universal indicator — a mixture of dyes that shows different colours at different pH values, used to demonstrate the alkaline nature of Group 1 metal hydroxides.
Vigour — the speed and energy with which a chemical reaction occurs; Group 1 reactions with water become more vigorous down the group.
Core concepts
Position in the periodic table and electron structure
Group 1 elements occupy the first column of the periodic table. All Group 1 metals have one electron in their outer shell, which explains their similar chemical properties.
The electron configurations are:
- Lithium (Li): 2,1
- Sodium (Na): 2,8,1
- Potassium (K): 2,8,8,1
- Rubidium (Rb): 2,8,18,8,1
- Caesium (Cs): 2,8,18,18,8,1
This single outer electron is easily lost during reactions, forming ions with a +1 charge (Li⁺, Na⁺, K⁺). This makes Group 1 metals very reactive compared to most other metals.
Key points about their structure:
- All have one electron in their highest energy level
- The number of occupied electron shells increases down the group
- They form ionic compounds by losing their single outer electron
- They exist as giant metallic structures in their elemental form
Physical properties
Group 1 metals share several distinctive physical properties that distinguish them from typical metals:
Softness:
- Can be cut with a knife
- Lithium is the hardest; caesium is the softest
- Softness increases down the group
- The shiny metal surface tarnishes rapidly when exposed to air
Low density:
- Lithium, sodium and potassium float on water
- Densities are lower than most other metals
- Lithium has a density of 0.53 g/cm³ (lower than water)
- Density generally increases down the group
Low melting and boiling points:
- Much lower than typical metals like iron or copper
- Lithium melts at 180°C; caesium at only 28°C
- Melting points decrease down the group
- This is due to weaker metallic bonding as atomic size increases
Appearance:
- Shiny and silver when freshly cut
- Quickly tarnish in air due to oxidation
- Stored under oil to prevent reaction with oxygen and water vapour
Reactions with water
Group 1 metals react vigorously with water, producing a metal hydroxide and hydrogen gas. This is one of the most important reactions you need to know for your exam.
General word equation:
alkali metal + water → metal hydroxide + hydrogen
General symbol equation:
2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)
(where M represents any Group 1 metal)
Specific examples:
Lithium + water:
2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)
Sodium + water:
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
Potassium + water:
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
Observations for lithium reacting with water:
- Floats on the water surface
- Fizzes steadily, producing hydrogen gas
- Moves around slowly on the water surface
- Gradually disappears as it reacts
- Solution becomes alkaline (pH 12-14)
Observations for sodium reacting with water:
- Floats and melts into a ball
- Fizzes vigorously
- Moves rapidly across the water surface
- May produce a yellow/orange flame (from sodium compounds)
- Reaction is faster than lithium
Observations for potassium reacting with water:
- Floats and melts immediately
- Fizzes very vigorously with flames
- Burns with a lilac/purple flame
- May ignite the hydrogen produced, causing small explosions
- Moves very quickly, often sparking
- Reaction is faster and more violent than sodium
The metal hydroxide dissolves in water, creating an alkaline solution. Universal indicator or litmus paper turns blue/purple, and the pH is typically 12-14.
Reactivity trend explained
Reactivity increases down Group 1. This means lithium is the least reactive and caesium is the most reactive (francium is radioactive and not studied at GCSE).
The trend is: Li < Na < K < Rb < Cs
Explanation of the trend:
When Group 1 metals react, they lose their single outer electron to form a +1 ion. The easier it is to lose this electron, the more reactive the metal.
Factors affecting how easily the outer electron is lost:
Distance from the nucleus:
- Down the group, atoms get larger (more electron shells)
- The outer electron is further from the positive nucleus
- The electrostatic attraction between the nucleus and outer electron decreases
- The electron is lost more easily
Electron shielding:
- Inner shell electrons repel the outer electron
- More shells = more shielding
- This reduces the effective nuclear charge experienced by the outer electron
- The outer electron is held less tightly
Nuclear charge:
- More protons down the group (stronger positive charge)
- However, this effect is outweighed by increased distance and shielding
- Distance and shielding are the dominant factors
Therefore, potassium loses its outer electron more easily than sodium, which loses it more easily than lithium. This explains why potassium reacts most vigorously with water.
Reactions with oxygen
Group 1 metals react with oxygen in the air, which is why they must be stored under oil.
General equation:
alkali metal + oxygen → metal oxide
Example with lithium:
4Li(s) + O₂(g) → 2Li₂O(s)
Example with sodium:
4Na(s) + O₂(g) → 2Na₂O(s)
Observations:
- Metals tarnish rapidly when freshly cut surface is exposed to air
- A dull oxide layer forms on the surface
- This layer is usually white or grey
- The reaction is faster for metals lower in the group
Some Group 1 metals can form peroxides or superoxides in excess oxygen, but this is beyond GCSE requirements.
Reactions with chlorine
Group 1 metals react vigorously with chlorine gas to form white crystalline metal chlorides.
General equation:
alkali metal + chlorine → metal chloride
Example with sodium:
2Na(s) + Cl₂(g) → 2NaCl(s)
Example with potassium:
2K(s) + Cl₂(g) → 2KCl(s)
These reactions produce ionic compounds. The metal chlorides dissolve in water to form neutral solutions (pH 7) because they are salts, not bases.
Worked examples
Example 1: Explaining the reactivity trend (4 marks)
Question: Explain why potassium reacts more vigorously with water than sodium.
Mark scheme answer:
Potassium has one more electron shell than sodium (1 mark).
The outer electron in potassium is further from the nucleus (1 mark).
There is more shielding by inner electrons in potassium (1 mark).
Therefore, the outer electron is lost more easily / the attraction between the nucleus and outer electron is weaker (1 mark).
Examiner note: You must mention distance/shells AND shielding to gain full marks. Simply stating "potassium is more reactive" scores zero marks.
Example 2: Writing balanced equations (3 marks)
Question: Lithium reacts with water. Write a balanced symbol equation for this reaction. Include state symbols.
Mark scheme answer:
2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)
(1 mark for correct formulae, 1 mark for balancing, 1 mark for all state symbols correct)
Common errors to avoid:
- Writing Li₂ instead of 2Li (lithium atoms exist as single atoms in equations)
- Forgetting to balance (common mistake is Li + H₂O → LiOH + H₂)
- Missing or incorrect state symbols
Example 3: Predicting observations (6 marks)
Question: A student adds a small piece of sodium to a trough of water containing universal indicator. Describe what the student would observe.
Mark scheme answer:
Sodium floats on the water (1 mark).
It fizzes/produces bubbles of gas (1 mark).
The sodium moves around on the surface of the water (1 mark).
The sodium melts into a ball/sphere (1 mark).
The sodium gradually gets smaller/disappears (1 mark).
The universal indicator changes colour to blue/purple, showing the solution is alkaline (1 mark).
Examiner note: Observations must be what you SEE, not explanations. "Produces hydrogen" is an explanation; "fizzes" is an observation.
Common mistakes and how to avoid them
Confusing Group 1 with Group 7: Group 1 are reactive metals that form positive ions; Group 7 are reactive non-metals that form negative ions. Always check which group the question asks about.
Getting the reactivity trend backwards: Some students incorrectly state that reactivity decreases down Group 1. Remember: reactivity INCREASES down the group (Li < Na < K). Use the mnemonic "Lower = Livelier" to remember that elements lower down are more reactive.
Incomplete explanations of reactivity: You must explain BOTH distance from nucleus AND shielding to gain full marks. Stating just one factor typically loses half the available marks.
Incorrect products of water reactions: The products are always a metal hydroxide and hydrogen, never a metal oxide. Sodium + water produces NaOH (not Na₂O) + H₂.
Confusing observations with explanations: "It fizzes" is an observation (what you see); "hydrogen is produced" is an explanation. In 6-mark observation questions, only describe what you would see, hear or smell.
Unbalanced equations: The most common error is writing Li + H₂O → LiOH + H₂. You must have 2Li + 2H₂O → 2LiOH + H₂ to balance the equation correctly.
Exam technique for "Atomic structure and the periodic table: properties of Group 1 (alkali metals)"
For "Explain" command words: You must give reasons, not just describe. When explaining the reactivity trend, always mention atomic radius/distance, shielding AND that the outer electron is lost more easily. A three-step answer (observation → reason → consequence) scores maximum marks.
For "Describe what you would see": Only give observations (what you could see with your eyes, or occasionally hear). Do not explain what's happening chemically. "Fizzes" scores a mark; "produces hydrogen gas" does not in an observation question.
Use correct chemical terminology: Write "alkali metal" not "Group 1 metal" when discussing their nature as metals that form alkaline solutions. Use "vigorous" or "vigour" to describe reaction speed, not vague terms like "fast" or "quickly."
Equation questions: State symbols are increasingly required at GCSE, so always include (s), (l), (g) and (aq) unless the question specifically says they're not needed. Check your equation balances by counting atoms on each side—2 marks often depends on correct balancing.
Quick revision summary
Group 1 metals (lithium, sodium, potassium) have one outer electron, making them highly reactive. They react with water to produce metal hydroxides and hydrogen, with increasing vigour down the group. Reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it's lost more easily. They're soft, have low melting points, and low densities. Always store under oil to prevent reaction with air and moisture. Remember: lower in the group = more reactive.