4.3 - 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.

Trends down the group

• Melting point decreases - Lithium melts at 180 °C, sodium at 98 °C and potassium at 63 °C.
• Density generally increases - The metals get denser going down the group (although potassium is slightly less dense than sodium).
• Reactivity increases - Lithium is the least reactive of the common alkali metals, and reactivity increases down the group to francium, the most reactive.

Predicting behaviour

• An element's properties can be predicted from the trends in the group.
• Given the melting points of lithium, sodium and potassium, rubidium can be predicted to have a melting point lower than potassium's (it melts at 39 °C).
• 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.

Why reactivity increases down Group 1

• Every alkali metal atom has one electron in its outer shell.
• To gain a stable full outer shell, a Group 1 atom loses that single outer electron to form an ion with a 1+ charge.
• The more easily an atom loses its outer electron, the more reactive the metal is.

Why it gets easier down the group

• Each element down the group has one more electron shell, so the atom is larger and the outer electron is further from the nucleus.
• The attraction between the positive nucleus and the negative outer electron weakens with distance.
• The extra inner electron shells shield the outer electron from the pull of the nucleus.
• Less energy is needed to remove the outer electron as you go down the group, so reactivity increases.

Forming ionic compounds

• Alkali metals react with non-metals by transferring their outer electron to form 1+ ions.
• The non-metal atoms gain electrons to form negative ions, producing an ionic compound (e.g. sodium + chlorine → sodium chloride).
• Group 1 ionic compounds are white solids that dissolve in water to give colourless solutions.

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

When heated in air, alkali metals burn to form white metal oxides (e.g. 4Li(s) + O₂(g) → 2Li₂O(s)).

Reaction with chlorine

Heated alkali metals burn vigorously in chlorine gas to produce white solid metal chlorides:

alkali metal + chlorine → metal chloride

For sodium: 2Na(s) + Cl₂(g) → 2NaCl(s)

Trend and storage

• All of these reactions become more vigorous down the group.
• Alkali metals are stored under oil to prevent them reacting with oxygen and moisture in the air.