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 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 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.

ElementObservations
Lithium, LiFizzes steadily, slowly becomes smaller until it disappears
Sodium, NaMelts to form a ball, fizzes rapidly, quickly becomes smaller until it disappears
Potassium, KQuickly melts to form a ball, burns violently with sparks and a lilac flame, disappears rapidly, often with a small explosion

Example

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

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 configurations 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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