4.1 - Group 1 - The Alkali Metals

The alkali metals and their trends

• The Group 1 elements are the metals in the first column of the periodic table: lithium, sodium, potassium, rubidium, caesium and francium.
• They are known as the alkali metals.
• Every atom has a single electron in its outer shell, which is why the whole group behaves in a similar way.

Physical properties

• Soft - Can be cut with a knife, exposing a shiny surface.
• Low density - Lithium, sodium and potassium are less dense than water, so they float.
• Low melting points - Sodium melts at 98 °C, which is very low for a metal.

Reactivity increases down the group

• Lithium is the least reactive of the common alkali metals, and reactivity increases down the group to francium, the most reactive.
• The further down the group, the more vigorous the reaction.
• Drop equal-sized pieces of lithium, sodium and potassium into water: lithium takes longest to react, sodium is quicker, and potassium is quickest.

Predicting behaviour

• An element's behaviour can be predicted from the elements above it.
• Rubidium and caesium sit below potassium, so their reactions are more violent: both react explosively with water, and caesium catches fire in air at room temperature.

The reactions of the alkali metals

Reaction with water

Alkali metals react vigorously with cold water to produce a metal hydroxide and hydrogen gas:

alkali metal + water → metal hydroxide + hydrogen

• For sodium: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
• The metal hydroxide dissolves to form an alkaline solution, turning universal indicator purple.

Reaction with oxygen

A freshly cut shiny surface quickly turns dull and grey (tarnishes) as the metal reacts with oxygen in the air to form a metal oxide:

alkali metal + oxygen → metal oxide

• Tarnishing happens faster further down the group.
• When heated in air, alkali metals burn to produce white solid metal oxides (e.g. 4Li(s) + O₂(g) → 2Li₂O(s)).

Storage

Alkali metals are stored under oil to prevent them reacting with oxygen and moisture in the air.