Group I Properties (Cambridge International IGCSE Chemistry)

Group I properties & trends: basics

Spec point — 8.2.1 · Describe the Group I alkali metals, lithium, sodium and potassium, as relatively soft metals with general trends down the group, limited to: decreasing melting point increasing density increasing reactivity

Group I properties & trends: basics

• The Group I metals are called the alkali metals

  • This is because they form alkaline solutions(with high pH values) when reacted with water

• Group I metals include:

  • Lithium, Li
  • Sodium, Na
  • Potassium, K

• They all contain just one electron in their outer shell

Physical properties of the Group I metals

• The Group I metals:

The alkali metals lie on the far left-hand side of the Periodic Table

• Are soft and easy to cut

  • They get softer down the Group

• Have shiny silvery surfaces when freshly cut

• Conduct heat and electricity

• Have low melting points compared to other metals

  • The melting point generally decreases as you move down the Group
  • Some would melt on a hot day

• Have low densities compared to other metals

Chemical properties of the Group I metals

• They react readily with oxygen and water vapour in air

  • This is why they are stored under oil, to stop them from reacting

• Group I metals will react similarly with water

  • They react to form an alkaline metal hydroxide solution and hydrogen gas

2M (s) + 2H₂O (l) → 2MOH (aq) + H₂(g)

• The Group I metals get more reactive as you move down the group

  • Only the first three metals are allowed in schools for demonstrations

Reactions of the Group I metals and water

Lithium

lithium + water → lithium hydroxide + hydrogen

2Li (s) + 2H2O (l) → 2LiOH (aq) + H2 (g)

• Observations:

  • Relatively slow reaction
  • Fizzing
  • Lithium moves on the surface of the water

Sodium

sodium + water → sodium hydroxide + hydrogen

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

• Observations:

  • More vigorous fizzing
  • Moves rapidly on the surface of the water
  • Dissolves quickly

Potassium

potassium + water → potassium hydroxide + hydrogen

2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

• Observations:

  • Reacts more vigorously than sodium
  • Burns with a lilac flame
  • Moves very rapidly on the surface
  • Dissolves very quickly

General trends of the Group I metals

• As you move down the group:

  • Melting point decreases

  • Density increases

    • There is an exception in the early part of the group

  • Reactivity (with water) increases

Predicting the Properties of Group I Elements

Spec point — 8.2.1 · Predict the properties of other elements in Group I, given information about the elements

• Knowing the reactions of elements at the top of the group allows you to predict the properties of other elements further down Group I

Properties of other alkali metals

• The reactivity of alkali metals increases down the group

• This means that rubidium, caesium and francium will react more vigorously with air and water than lithium, sodium and potassium

  • Lithium will be the least reactive metal at the top of the group
  • Francium will be the most reactive at the bottom

• Francium is rare and radioactive so is difficult to confirm predictions

• For example the reactions with water can be predicted:

Predicting the reaction with water

Rubidium

• Predicted observations:

  • Very violent reaction
  • May explode with sparks

Caesium

• Predicted observations:

  • Extremely violent reaction
  • Can explode due to rapid production of heat and hydrogen

Francium

• Predicted observations:

  • Extremely reactive
  • Difficult to confirm experimentally due to rarity and radioactivity

You can use any given data for Group 1 elements to identify trends and make predictions about other elements in the group.