Group 1: the alkali metals
Group 1 holds lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and caesium (Cs). They are called alkali metals because they react with water to make alkalis (soluble bases).
- One outer electron: electronic configuration ends in ns¹ (Na: 2, 8, 1).
- They lose that electron easily and form M⁺ ions, so their compounds are ionic.
- Soft: sodium can be cut with a knife. Shiny when freshly cut, but it soon goes dull in air.
- Low density: Li, Na and K float on water.
- Low melting points that fall down the group.
Trends down the group
Atomic size increases, so the outer electron is further from the nucleus and held less tightly. Ionisation energy falls and reactivity increases: Li reacts gently with water, Na fizzes, K catches fire with a lilac flame.
Because they react with air and water, Na and K are stored under kerosene or paraffin oil.
Reactions of sodium
With water
2Na + 2H₂O → 2NaOH + H₂↑. The sodium floats, melts into a ball (the reaction gives out heat), fizzes and moves about. A drop of phenolphthalein turns pink because NaOH is an alkali.
With oxygen
- Slowly, at room temperature: 4Na + O₂ → 2Na₂O (sodium oxide, white).
- Burnt in plenty of air: 2Na + O₂ → Na₂O₂ (sodium peroxide, pale yellow).
Both react with water. Na₂O + H₂O → 2NaOH. Sodium peroxide also gives oxygen: 2Na₂O₂ + 2H₂O → 4NaOH + O₂↑, and with carbon dioxide: 2Na₂O₂ + 2CO₂ → 2Na₂CO₃ + O₂. That is why Na₂O₂ has been used to supply oxygen in submarines and breathing kits.
With chlorine
2Na + Cl₂ → 2NaCl (common salt), with a bright yellow flame.
Sodium carbonate and sodium hydrogencarbonate
| Na₂CO₃ (washing soda when hydrated: Na₂CO₃·10H₂O) | NaHCO₃ (baking soda) | |
|---|---|---|
| Solubility | More soluble | Less soluble |
| Solution | Quite strongly alkaline | Weakly alkaline |
| On heating | Does not decompose | 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑ |
| With acid | Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ | NaHCO₃ + HCl → NaCl + H₂O + CO₂ (faster) |
| Uses | Glass, soap, softening hard water | Baking, antacids, fire extinguishers |
To tell them apart, heat each solid and pass the gas into lime water: only NaHCO₃ turns it milky. They can be changed into each other: Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃.
Flame tests and the neighbours of group 1
Flame tests
Dip a clean platinum or nichrome wire in the salt and hold it in a hot, nearly colourless flame. Heat lifts electrons to higher levels; when they fall back they give out light of a set colour.
- Li: crimson red · Na: bright yellow · K: lilac (see it through blue cobalt glass to hide sodium's yellow) · Ca: brick red · Ba: pale green · Cu: blue-green.
Fireworks use these colours.
Group 2 and aluminium in brief
Magnesium and calcium (group 2) have two outer electrons, form M²⁺ ions and are less reactive than group 1. Dissolved Ca²⁺ and Mg²⁺ make water hard: soap forms scum. Temporary hardness (from hydrogencarbonates) is removed by boiling; permanent hardness (sulfates, chlorides) by washing soda or ion exchange.
Aluminium (group 13) forms Al³⁺. Its oxide and hydroxide are amphoteric: they react with both acids and alkalis, e.g. Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O and Al(OH)₃ + NaOH → Na[Al(OH)₄].
In living things
Na⁺ and K⁺ carry nerve signals and control water balance; Ca²⁺ builds bones and teeth; Mg²⁺ sits at the centre of chlorophyll. Bananas and coconut water are rich in potassium.
Try it at home
Put a spoon of baking soda in a glass, add lemon juice or vinegar and watch the CO₂ fizz. Then sprinkle a pinch of salt into a gas flame (with an adult) and look for the yellow sodium colour.
Key formulas and definitions
- 2M + 2H₂O → 2MOH + H₂↑ (M = Li, Na, K…)
- 4Na + O₂ → 2Na₂O; 2Na + O₂ → Na₂O₂
- 2Na₂O₂ + 2H₂O → 4NaOH + O₂↑
- 2NaHCO₃ →(heat) Na₂CO₃ + H₂O + CO₂↑
- Flame colours: Li red, Na yellow, K lilac, Ca brick red, Ba green, Cu blue-green
Worked examples
1. Why does potassium react with water more vigorously than sodium?
K has one more electron shell than Na, so its outer electron is further from the nucleus and more shielded. It is lost more easily, so K reacts faster.
2. 4.6 g of sodium reacts completely with water. What mass of hydrogen is produced? (Na = 23, H = 1)
2Na → H₂. Moles Na = 4.6 / 23 = 0.2 mol, so moles H₂ = 0.1 mol. Mass = 0.1 × 2 = 0.2 g.
3. 8.4 g of NaHCO₃ is heated fully. What volume of CO₂ forms at STP? (NaHCO₃ = 84 g/mol, 22.4 L/mol)
2NaHCO₃ → CO₂. Moles NaHCO₃ = 0.1 mol, so CO₂ = 0.05 mol. Volume = 0.05 × 22.4 = 1.12 L.
4. Two white powders are Na₂CO₃ and NaHCO₃. Give one simple test to tell them apart.
Heat each and bubble the gas through lime water. NaHCO₃ gives CO₂ which turns lime water milky; Na₂CO₃ gives no gas.
Common mistakes
- Writing the sodium + water product as Na₂O. The product is NaOH and H₂.
- Mixing up Na₂O (oxide, O²⁻) and Na₂O₂ (peroxide, O₂²⁻). Peroxide forms when sodium burns in plenty of air.
- Saying Na₂CO₃ gives CO₂ on heating. Only NaHCO₃ decomposes.
- Thinking reactivity decreases down group 1. For metals it increases, because the outer electron is lost more easily.