Group 1: the alkali metals

Why does potassium burst into flame on water?

Drop lithium into water and it fizzes steadily. Sodium races around the surface. Potassium reacts violently. All three are in group 1, so why do they behave so differently?

Every group 1 element has one electron in its outer shell (ns¹). It is removed easily, so these metals are very reactive. Hydrogen sits at the top of the group but is a gas and a non-metal.

Going down the group the atoms get bigger, so the outer electron is lost more easily and reactivity rises. With water, lithium reacts steadily, sodium vigorously and potassium violently; each gives hydrogen and a soluble hydroxide. Their reactions with chlorine also get more vigorous down the group.

Bigger atoms also weaken the attraction between atoms, so the metals get softer and their melting points fall: 180 °C for lithium down to 28 °C for caesium (Table 9.1). Mass grows faster than volume, so density generally rises, though potassium (0.86 g/cm³) is lighter than sodium (0.97 g/cm³).

Down group 1: bigger atoms, easier electron loss, more reactive, softer, lower melting point.

Key terms

Alkali metal

Naming the group.

Reactivity

Comparing Li, Na and K with water.

Melting point

Showing weaker attraction down the group.

Hydroxide

Product of the water reaction.

Short questions with model answers

1. Q1. Write the equation for sodium reacting with water and name the products.

   • Pattern
   • 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

   Sodium hydroxide (dissolved) and hydrogen gas.

2. Q2. Predict how potassium reacts with chlorine compared with sodium. (Short question ii, p. 127)

   • Position
   • Trend
   • 2K(s) + Cl₂(g) → 2KCl(s)

   Potassium reacts violently with chlorine, more vigorously than sodium, giving KCl.

Common mistakes

• ✗ Melting point rises down group 1 because atoms get heavier.

  ✓ It falls: bigger atoms attract each other more weakly (Table 9.1).

• ✗ Hydrogen is an alkali metal because it is in group 1.

  ✓ Hydrogen is a gas and a non-metal (p. 120).

• ✗ Density rises at every step down group 1.

  ✓ Generally, but K (0.86) is lighter than Na (0.97) in Table 9.1.

MCQs

1. 1. Which property is correct for group 1 elements? (Exercise x, p. 127)

   1. (a) Low catalytic activity
   2. (b) High density
   3. (c) Low electrical conductivity
   4. (d) High melting point
   Show answer

   (a) They are soft, light, low-melting metals that conduct well; catalysts are transition metals (p. 123).

2. 2. Which group 1 metal reacts most violently with water?

   1. (a) Lithium
   2. (b) Sodium
   3. (c) Potassium
   4. (d) Hydrogen
   Show answer

   (c) Of these, K is lowest in the group (p. 120).

3. 3. Why does an alkali metal get easier to cut down the group? (Short question i, p. 127)

   1. (a) Atoms get smaller
   2. (b) Attraction between bigger atoms is weaker
   3. (c) Density falls
   4. (d) They lose their electron
   Show answer

   (b) p. 120: interatomic attraction weakens, so softer.

4. 4. Which gas is given off when lithium reacts with water?

   1. (a) Oxygen
   2. (b) Hydrogen
   3. (c) Chlorine
   4. (d) Carbon dioxide
   Show answer

   (b) 2Li + 2H₂O → 2LiOH + H₂ (p. 120).

5. 5. Which group 1 metal in Table 9.1 has the lowest melting point?

   1. (a) Li
   2. (b) Na
   3. (c) K
   4. (d) Cs
   Show answer

   (d) Cs melts at 28 °C.

Quick revision

• Group 1 metals have one outer electron (ns¹) that is lost easily.
• Reactivity with water and chlorine increases down the group: Li steady, Na vigorous, K violent.
• Down the group the metals get softer and melt at lower temperatures; density generally rises.