What are the key properties of Group I alkali metals?

Group I alkali metals are soft, highly reactive, and have low melting and boiling points.

Group I alkali metals, which include lithium, sodium, potassium, rubidium, caesium, and francium, are known for their distinctive properties. They are soft enough to be cut with a knife and have a shiny appearance when freshly cut. However, they tarnish quickly when exposed to air due to their high reactivity. This reactivity is due to the single electron in their outermost shell, which they readily lose to achieve a stable electron configuration. This makes them highly reactive, especially with water, and they must be stored under oil to prevent reaction with air or moisture.

These metals have low melting and boiling points compared to most other metals, and these decrease down the group. For example, lithium melts at 180°C and boils at 1342°C, while francium, the heaviest alkali metal, melts at 27°C and boils at 677°C. This trend is due to the increasing atomic size and decreasing ionisation energy down the group.

Alkali metals also have low densities, with lithium, sodium, and potassium being less dense than water. They are good conductors of heat and electricity due to the free electrons in their structure. When they react with water, they produce alkaline solutions, hence their name 'alkali' metals. They also form ionic compounds with non-metals, in which they carry a positive charge.

In terms of their chemical behaviour, alkali metals are very predictable. They all react with halogens to form salts, and with water to produce hydrogen gas and alkaline solutions. The reactivity of the alkali metals increases down the group, with francium being the most reactive. This is because the outermost electron becomes more easily lost as the atomic size increases, reducing the ionisation energy.