Chemical properties of the alkali metals

Atoms of group 1 elements all have one electron in their outer shell. This means that the alkali metals all have similar chemical properties.

Reactions with group 7 elements

The alkali metals react with group 7 elements. For example, lithium reacts with chlorine to form lithium chloride:

lithium + chlorine → lithium chloride

2Li(s) + Cl2(g) → 2LiCl(s)

A similar reaction takes place with the other elements of group 7.

Reactions with oxygen

The alkali metals react with oxygen in the air. For example, sodium burns in air with a yellow flame, forming sodium oxide:

sodium + oxygen → sodium oxide

4Na(s) + O2(g) → 2Na2O(s)

Group 1 metals are usually stored under oil to prevent this reaction.

Reactions with water

The alkali metals react with water to produce a metal hydroxide and hydrogen. For example, sodium reacts with water:

Sodium + water → sodium hydroxide + hydrogen

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Sodium hydroxide is an alkali. It is a base that dissolves in water to form an alkaline solution. This solution:

Hydrogen gas is also produced. With very reactive metals such as potassium, the energy of the reaction causes the hydrogen to burn as the reaction takes place.

Question

Write the word equation and balanced equation for the reaction of potassium with water.

Potassium + water → potassium hydroxide + hydrogen

2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)

Reactions compared

The reaction of potassium with water gives a lilac flame
The reaction of potassium with water gives a lilac flame

The table shows observations when lithium, sodium and potassium are added to water. Notice that the reactivity of these metals increases going down the group. This pattern is seen with all reactions of group 1 elements. For example, the reaction of caesium with chlorine is more vigorous than the reaction of potassium with chlorine.

ElementObservation
Lithium, LiFizzes steadily
Lithium, LiSlowly becomes smaller until it disappears
Sodium, NaMelts to form a ball
Sodium, NaFizzes rapidly
Sodium, NaQuickly becomes smaller until it disappears
Potassium, KQuickly melts to form a ball
Potassium, KBurns violently with sparks and a lilac flame
Potassium, KDisappears rapidly, often with a small explosion
Question

Rubidium is placed below potassium in group 1. Predict what is seen when rubidium is added to water.

Rubidium should melt very quickly. It should burn very violently, and disappear almost instantly with an explosion.

Explaining the trend

When an element in group 1 takes part in a reaction, its atoms lose their outer electron and form positively charged ions, called cations. The more easily these cations form, the more reactive the metal.

Table showing electronic structures of group 1 elements, lithium, sodium and potassium. Group 1 elements have similar properties and reactions as they all have one electron in their outer shell.

The reactivity of group 1 elements increases as you go down the group because:

  • the atoms become larger
  • the outer electron becomes further from the nucleus
  • the force of attraction between the nucleus and the outer electron decreases
  • the outer electron is lost more easily

The periodic table, containing all the known elements, arranged in groups and periods

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