S4 Chemistry Practice Book · Unit 6: Trends in chemical properties of group 1 elements and their compounds

Unit 6 Trends in chemical properties of group 1 elements and their compounds

Key unit competence: Interpret the periodic trends in the properties of group 1 elements and their compounds to explain their chemical reactivity and uses.

Key ideas

Group 1 (alkali metals): Li, Na, K, Rb, Cs, Fr; configuration ns¹. They are soft, low-density metals with low melting points that decrease down the group; stored under oil; found as compounds (e.g. NaCl in rock salt and sea water, KCl).
Reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so ionisation energy decreases. They always form M⁺ ions.
With oxygen: Li forms Li₂O (oxide), Na forms Na₂O₂ (peroxide) in excess oxygen, K, Rb and Cs form superoxides (KO₂). With water: 2M + 2H₂O → 2MOH + H₂; Li reacts steadily, Na melts and darts, K catches fire with a lilac flame, Rb and Cs explode. With halogens: 2Na + Cl₂ → 2NaCl.
Oxides and hydroxides are strongly basic; hydroxides are soluble strong alkalis whose solubility and basic strength increase down the group.
Thermal stability: group 1 carbonates (except Li₂CO₃) do not decompose with a Bunsen burner; Li₂CO₃ → Li₂O + CO₂. Nitrates: 4LiNO₃ → 2Li₂O + 4NO₂ + O₂; other group 1 nitrates give nitrites: 2NaNO₃ → 2NaNO₂ + O₂. Lithium is anomalous because Li⁺ is very small and polarising.
Flame tests: Li crimson red, Na yellow-orange, K lilac. Uses: Na in street lamps and as coolant in nuclear reactors, Li in batteries, NaCl in food and chlor-alkali industry, NaOH in soap and paper, KCl and KNO₃ in fertilisers, NaHCO₃ in baking and antacids.

Common misconceptions

"Reactivity of group 1 metals decreases down the group like the halogens." No: It increases down group 1 because the outer electron is lost more easily.
"All group 1 metals form the oxide M₂O in excess oxygen." No: Sodium forms a peroxide and potassium a superoxide in excess oxygen.
"All group 1 nitrates decompose like lithium nitrate." No: Only lithium nitrate forms an oxide and NO₂; the others form nitrites and oxygen.

Worked examples

Example 1. Describe and explain the reactions of lithium, sodium and potassium with water.

Solution

Lithium: floats, fizzes steadily, slowly gets smaller: 2Li + 2H₂O → 2LiOH + H₂.

Sodium: floats, melts into a ball, darts around, fizzes rapidly.

Potassium: reacts violently, hydrogen ignites with a lilac flame.

Reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily.

Example 2. Compare the effect of heat on lithium nitrate and sodium nitrate, and explain the difference.

Solution

Lithium nitrate: 4LiNO₃ → 2Li₂O + 4NO₂ + O₂ (brown fumes).

Sodium nitrate: 2NaNO₃ → 2NaNO₂ + O₂ (no brown fumes).

Li⁺ is very small with a high charge density; it polarises the nitrate ion, breaking it down further.

This is one of the anomalous properties of lithium (similar to magnesium: diagonal relationship).

Practice questions

Write the configuration of potassium (Z = 19).
Show answer
1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.
Common mistake: Writing 3d¹.
Give the flame colours of Li, Na and K.
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Crimson red, yellow, lilac.
Common mistake: Mixing up Li and K.
Write the equation for lithium burning in oxygen.
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4Li + O₂ → 2Li₂O.
Common mistake: Writing LiO.
Why do alkali metal hydroxides become stronger bases down the group?
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Larger cations hold OH⁻ less strongly, and solubility increases.
Common mistake: Saying they gain protons.
Give two uses of sodium compounds.
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NaCl (food, chlorine), NaOH (soap), NaHCO₃ (baking, antacid), Na₂CO₃ (glass, softening water) (any two).
Common mistake: Saying sodium metal is eaten.
Explain why caesium is more reactive than sodium.
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Caesium's outer electron is much further from the nucleus and more shielded: lower ionisation energy.
Common mistake: Saying caesium has fewer protons.
Write the equation for sodium reacting with chlorine.
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2Na + Cl₂ → 2NaCl.
Common mistake: Writing NaCl₂.
State one anomalous property of lithium.
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Its carbonate decomposes on heating / nitrate gives NO₂ / forms nitride with nitrogen / compounds more covalent (any one).
Common mistake: Saying it reacts most violently with water.
Why does the melting point decrease down group 1?
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Larger ions and the same single delocalised electron give weaker metallic bonding.
Common mistake: Saying atomic mass decreases.
Calculate the volume of hydrogen at STP when 3.9 g of potassium (Ar 39) reacts with water.
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n(K) = 0.1 mol; H₂ = 0.05 mol; V = 1.12 dm³.
Common mistake: Using a 1 : 1 ratio.

Matching and drag-and-drop

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. Which group 1 metal is used in rechargeable batteries for phones?	a. lilac
2. Group 1 hydroxides are:	b. strong, soluble alkalis
3. Potassium burns with a flame colour that is:	c. Lithium
4. Lithium gives a flame colour that is:	d. crimson red

Show answer

1 → c (Lithium) · 2 → b (strong, soluble alkalis) · 3 → a (lilac) · 4 → d (crimson red)

Lithium-ion batteries.
They fully ionise in water.
Characteristic flame test.
Lithium compounds give red flames.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
increases · sodium carbonate · stays the same · sodium nitrite and oxygen
1. A white solid gives a yellow flame and, with dilute HCl, a gas that turns limewater milky. It is .
2. Heating sodium nitrate gives .
3. Down group 1, reactivity .
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1: sodium carbonate · 2: sodium nitrite and oxygen · 3: increases
- Step 1. Yellow flame: sodium.
  Step 2. CO₂ with acid: carbonate.
- 2NaNO₃ → 2NaNO₂ + O₂.
- Outer electron is lost more easily.

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. In excess oxygen, sodium forms mainly:	a. street lamps
2. Sodium vapour is used in:	b. ns¹
3. Group 1 elements have outer electron configuration:	c. Lithium
4. Which group 1 metal has the highest melting point?	d. sodium peroxide, Na₂O₂

Show answer

1 → d (sodium peroxide, Na₂O₂) · 2 → a (street lamps) · 3 → b (ns¹) · 4 → c (Lithium)

Peroxide ion O₂²⁻.
It gives yellow light.
One electron in the outer s orbital.
Melting point decreases down the group.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
lithium oxide, nitrogen dioxide and oxygen · a fuel · M⁺ · a fertiliser
1. The ion formed by all group 1 metals is .
2. Potassium chloride is used mainly as .
3. Lithium nitrate decomposes on heating to give .
Show answer
1: M⁺ · 2: a fertiliser · 3: lithium oxide, nitrogen dioxide and oxygen
- They lose one electron.
- It provides potassium for plants.
- Lithium behaves like group 2 nitrates.

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