1.1.4Building Blocks for Understanding
Group 1 — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 7 parts Lesson 5 of 14
Part 1 of 5Physical properties
Soft metals
- Soft metals Lithium, sodium and potassium can be cut with a knife and have shiny surfaces that tarnish quickly in air.
- They are stored under oil to keep air and moisture away.
Low density and melting points
- Low density and melting points Lithium, sodium and potassium float on water.
- They have lower melting points than most metals, and these fall further down the group.
One outer electron
- One outer electron Every Group 1 atom has a single electron in its outer shell, so each forms a 1+ ion.
- Their compounds are usually white solids that dissolve in water.
Part 2 of 5Reactions with water
1
Drop a small piece of the metal onto water in a safety-screened trough, wearing eye protection.
2
Lithium fizzes steadily, sodium melts into a ball and moves, and potassium ignites with a lilac flame.
3
The products are a metal hydroxide and hydrogen gas.
4
Example: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g).
5
The solution is alkaline, so universal indicator turns purple, which is why the group is called the alkali metals.
6
A lighted splint held at the mouth of a test tube of the gas gives a squeaky pop, showing that hydrogen is made.
Part 3 of 5Other reactions
| Reactant | Product | Example |
|---|---|---|
| Oxygen | Metal oxide | 4Li + O2 → 2Li2O |
| Chlorine | White metal chloride | 2Na + Cl2 → 2NaCl |
| Water | Metal hydroxide and hydrogen | 2K + 2H2O → 2KOH + H2 |
Part 4 of 5Why reactivity increases down the group
1More shells
- More shells Atoms get larger going down the group.
2Outer electron farther away
- Outer electron farther away It is held less strongly by the nucleus because of greater distance and shielding.
3Easier to lose
- Easier to lose Less energy is needed to remove the electron, so the metal reacts more vigorously.
4More vigorous reaction
- More vigorous reaction Rubidium and caesium react explosively with water.
Part 5 of 5Comparing the first three alkali metals
Reaction with water
Lithium
- Lithium Fizzes steadily and moves slowly across the surface without melting.
Sodium
- Sodium Melts into a silvery ball because the reaction releases enough heat, and darts around the surface.
Potassium
- Potassium Reacts so quickly that the hydrogen ignites with a lilac flame.
Key terms
| Term | Meaning |
|---|---|
| Alkali metal | A Group 1 element, reacting with water to give an alkaline solution. |
| Hydroxide | A compound containing the OH- ion; its solutions are alkaline. |
| Reactivity | How readily an element takes part in chemical reactions. |
| Shielding | The reduction of nuclear attraction on outer electrons caused by inner shells. |
| Tarnish | A dull layer formed when a metal surface reacts with air. |
| Universal indicator | A mixture of dyes that shows the pH of a solution by its colour. |
Avoid these mistakes
1
Saying reactivity decreases down the group; it increases.
2
Writing the product of an alkali metal with water as an oxide instead of a hydroxide.
3
Forgetting that hydrogen gas is also produced, leaving the equation unbalanced.
4
Explaining the trend using the number of outer electrons; it is the same for all, so the distance and shielding are the cause.
5
Saying the metals sink in water. Lithium, sodium and potassium are less dense than water.
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