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2.1Inorganic Chemistry

Group 1 (Alkali Metals) — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 7 of 23 Unit quiz

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Lesson summary

Group 1: The Alkali Metals — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe the trends in physical and chemical properties of lithium, sodium and potassium and explain them using electronic structure.Group 1 elements (lithium, sodium, potassium, rubidium, caesium, francium) are …

  • one electron in its outer shell
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Outer electron

Part 1 of 6Position and electronic configuration

Group 1 elements (lithium, sodium, potassium, rubidium, caesium, francium) are known as the alkali metals because they react with water to form alkaline hydroxide solutions. Every Group 1 atom has one electron in its outer shell: Li is 2,1; Na is 2,8,1; K is 2,8,8,1. Elements in the same group have similar chemical properties because they have the same number of outer electrons.

Each atom loses its single outer electron to form a 1+ ion with a stable noble-gas configuration, for example Na → Na+ + e-. The compounds formed are ionic, white solids that dissolve in water to give colourless solutions.

Part 2 of 6Physical properties and trends down the group

The alkali metals are unusual metals

  • they are soft (cut with a knife), have low melting points and low densities (lithium, sodium and potassium float on water).
  • Fresh cut surfaces are shiny but tarnish quickly in air as they form an oxide, so they are stored under oil.

PropertyTrend down the group
Melting and boiling pointDecrease
SoftnessIncreases (metals become softer)
DensityGenerally increases (K is less dense than Na, an exception)
ReactivityIncreases

Reactivity increases because as you go down the group the outer electron is further from the nucleus and is more shielded by inner shells. The attraction between the nucleus and this electron is weaker, so it is lost more easily.

Part 3 of 6Reactions with water, oxygen and chlorine

All alkali metals react with water to give hydrogen and the metal hydroxide: 2Na + 2H2O → 2NaOH + H2. The solution turns universal indicator purple (pH 13–14) showing a strong alkali.

Lithium
Lithium fizzes steadily, moves on the surface, dissolves slowly.
Sodium
Sodium melts into a ball (the reaction is exothermic and sodium has a low melting point), darts around and fizzes rapidly; hydrogen may ignite with a yellow flame.
Potassium
Potassium reacts very quickly and the hydrogen burns with a lilac flame, often with a small explosion.

The similarity of these reactions (the same products, a gas given off and an alkaline solution) is the evidence for treating them as a family, while the increasing vigour shows the trend in reactivity.

With oxygen they form oxides (4Li + O2 → 2Li2O), and with chlorine they form chlorides (2Na + Cl2 → 2NaCl), burning with a characteristic flame colour: lithium red, sodium yellow, potassium lilac.

Part 4 of 6Predicting properties and exam application

Questions often ask you to predict rubidium or caesium.

1
Rubidium is more reactive than potassium: it would react explosively with water, melt, and have a lower melting point and be softer.
2
The equation follows the same pattern: 2Rb + 2H2O → 2RbOH + H2.
3
Always justify using the outer electron, distance and shielding.
4
Safety: use small pieces, safety screen, goggles and tongs because the metals are corrosive and hydrogen is flammable.

Part 5 of 6Worked reasoning: linking structure to behaviour

To score full marks, chain the ideas in order: (1) all Group 1 atoms have one outer electron, so they react in the same way and form 1+ ions; (2) going down, atoms have more shells, so the outer electron is further away and shielded by more inner electrons; (3) the attraction from the nucleus is weaker; (4) the electron is lost more easily, so the metal is more reactive. Each step is a separate marking point.

The same reasoning explains the physical trends: bigger atoms mean weaker metallic bonding between the positive ions and the sea of delocalised electrons, so melting points fall and the metals get softer.

Worked reasoning: linking structure to behaviour
Compounds of alkali metals are used widely: sodium chloride in food and in electrolysis, sodium hydroxide in soap making, and potassium compounds in fertilisers.
Because the ions are 1+ and colourless, solutions of their salts are colourless, and the metals can be identified by the flame colour they give: lithium crimson red, sodium golden yellow and potassium lilac.

Part 6 of 6Worked examples

  1. Write a balanced equation for the reaction of potassium with water and state the pH of the solution. [3]3 marks

  2. Explain why potassium is more reactive than lithium. [3]3 marks

  3. Describe two observations when sodium is placed in water. [2]2 marks

  4. Predict how the reaction of rubidium with water differs from that of sodium. [2]2 marks

Key terms

TermMeaning
Alkali metalsGroup 1 elements that form alkaline solutions with water
Outer electronThe electron in the highest occupied shell that determines chemical properties
ShieldingInner electrons reduce the attraction between the nucleus and outer electron
HydroxideCompound containing OH− ions, forming alkaline solutions
Flame colourCharacteristic colour of a metal ion in a flame: Li red, Na yellow, K lilac
GroupA vertical column in the periodic table with the same number of outer electrons

Avoid these mistakes

1
Saying the metals are harder further down — they get softer.
2
Thinking the outer electron is held more strongly in larger atoms — it is held more weakly because of distance and shielding.
3
Forgetting the hydroxide productthe solution is alkaline, not neutral.
4
Saying lithium, sodium and potassium sink in water — they float (rubidium and caesium sink).

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