8.2The Periodic Table
Group I properties — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 9 parts Lesson 2 of 5 1 quiz
Lesson summary
8.2 Group I properties — Notes — IGCSE Chemistry notes: Describe the physical and chemical properties of the Group I alkali metals, their reactions with water and their trends down the group.Group I metals: lithium, sodium, potassium, rubidium, caesium and francium are called the…
- Reactivity
- Group I metals: lithium, sodium, potassium, rubidium, caesium and francium are called the alkali metals
- They form alkaline solutions with high pH when they react with water
- Each atom has one electron in its outer shell, which it loses to form a 1+ ion such as Na+
- Soft: they are easy to cut with a knife and get softer down the group
- Shiny silvery surfaces when freshly cut
Section 1 of 9The alkali metals
The alkali metals
Group I metals: lithium, sodium, potassium, rubidium, caesium and francium are called the alkali metals.
They form alkaline solutions with high pH when they react with water.
Each atom has one electron in its outer shell, which it loses to form a 1+ ion such as Na+.
Section 2 of 9Physical properties
1
Soft: they are easy to cut with a knife and get softer down the group.
2
Shiny silvery surfaces when freshly cut.
3
Good conductors of heat and electricity.
4
Low melting points compared with other metals, decreasing down the group.
5
Low densities compared with other metals, so lithium, sodium and potassium float on water.
Section 3 of 9General trends down the group
1
Melting point decreases.
2
Density generally increases, with an exception early in the group, as potassium is less dense than sodium.
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Reactivity increases, so lithium is the least reactive and francium the most reactive.
4
Explanation of reactivity
- Atoms have more shells down the group.
- The outer electron is further from the nucleus.
- The attraction to the nucleus is weaker.
- The outer electron is lost more easily, so reactivity increases.
Section 4 of 9Chemical properties
1
Reaction with air: they react readily with oxygen and water vapour in air, tarnishing cut surfaces, which is why they are stored under oil.
2
Reaction with water: 2M(s) + 2H2O(l) → 2MOH(aq) + H2(g), forming an alkaline metal hydroxide solution and hydrogen gas.
3
Universal indicator turns purple in the solution formed.
4
Safety: only lithium, sodium and potassium are allowed in school demonstrations.
Section 5 of 9Reactions with water
| Metal | Observations with water |
|---|---|
| Lithium | Relatively slow reaction; steady fizzing; moves on the surface of the water |
| Sodium | More vigorous fizzing; melts into a ball and moves rapidly on the surface; dissolves quickly |
| Potassium | Reacts more vigorously than sodium; burns with a lilac flame; moves very rapidly; dissolves very quickly |
| Rubidium (predicted) | Very violent reaction, may explode with sparks |
| Caesium (predicted) | Extremely violent reaction, can explode due to rapid production of heat and hydrogen |
| Francium (predicted) | Extremely reactive, difficult to confirm because it is rare and radioactive |
Section 6 of 9Predicting properties
Predicting properties
Use the reactions of the elements at the top of the group to predict those further down.
Rubidium, caesium and francium react more vigorously with air and water than lithium, sodium and potassium.
Using data: put the elements in order of position in the group, then look for general patterns.
Section 7 of 9More from the specification
More from the specification
Alkali metal shiny solid appearance
- Group 1 elements are soft, low-density solid metals that tarnish rapidly in air when freshly cut with a knife.
Periodic
- Describing properties that repeat at regular intervals along the table
Noble gases
- The Group 0 elements, which are unreactive monatomic gases
Physical Trends in Alkali Metals
- Group I alkali metals are soft, low-density metals with melting points and boiling points that decrease progressively down the group.
- (Li, Na, K, Rb, Cs)
Vigorous Reactivity with Water
- Group I metals react vigorously with cold water to produce an alkaline metal hydroxide solution and effervescing hydrogen gas (e.g. 2Na + 2H2O → 2NaOH + H2).
Increasing Chemical Reactivity Down Group I
- As atomic radius increases down the group, the single valence electron is further from the nucleus and more shielded, requiring less energy to be lost to form a unipositive cation.
Transition elements
- The central block of metals between Group II and Group III.
Halogens
- Group 17 non-metals that form diatomic molecules.
Halogen
- A Group VII non-metal with seven outer electrons.
Aqueous
- Dissolved in water.
Displacement
- A reaction in which a more reactive element takes the place of a less reactive one in a compound.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Alkali metals | the Group I metals, which form alkaline solutions with water. |
| Reactivity | how readily an element takes part in chemical reactions. |
| Tarnish | the dull layer formed when a metal reacts with air. |
| Hydroxide | a compound containing the OH− ion, formed by Group I metals with water. |
| Francium | the most reactive Group I metal, rare and radioactive. |
Section 9 of 9Exam technique
1
Saying sodium reacts with water to make sodium oxide: it forms sodium hydroxide and hydrogen.
2
Saying reactivity decreases down Group Iit increases.
3
Saying alkali metals are hardthey are soft and easy to cut.
4
Forgetting that the alkali metals are stored under oil because they react with air and water vapour.
Marked practiceQuizzes for this lesson
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