6.1 - 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.

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

• Alkali metals react vigorously with cold water to produce:
  • Hydrogen gas (seen as fizzing).
  • A metal hydroxide, which dissolves to form an alkaline solution.
• Universal indicator added to the water turns from green to purple.

Word and balanced equations

alkali metal + water → metal hydroxide + hydrogen

For sodium:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Lithium and potassium react in the same way, producing lithium hydroxide and potassium hydroxide.

Observations with cold water