Physical properties of Group I metals

Lithium, sodium and potassium are Group I metals, also called alkali metals. They are relatively soft compared with many other metals and can be cut with a knife.

Trends down Group I

In the order lithium, sodium, potassium, the elements are progressively further down Group I. Their physical properties show two useful trends:

• Melting point decreases: potassium has a lower melting point than sodium, which has a lower melting point than lithium.
• Density generally increases: elements further down the group tend to have more mass in the same volume, although the trend has exceptions. For example, potassium is less dense than sodium.

Predicting Group I properties

An element’s position in Group I can be used with information about other members to predict its likely properties. For example, an element below potassium would be expected to have a lower melting point than potassium and, following the general trend, a higher density.

These are general trends, not exact values or rules without exceptions. A prediction gives a likely property, but measurement is needed to establish the value.

Reactivity trend of Group I metals

Group I metals become more reactive down the group. Reactivity describes how readily an element undergoes chemical reactions.

Lithium, sodium and potassium occur in that order down Group I. Sodium is more reactive than lithium, and potassium is more reactive than sodium. Potassium is therefore the most reactive of the three.

Predicting Group I reactivity

An element’s position in Group I can be used to predict its reactivity relative to other Group I metals: an element lower down is expected to be more reactive than one above it. For example, if supplied information shows that rubidium is below potassium, rubidium can be predicted to be more reactive than potassium.

This trend predicts the relative order of reactivity; it does not by itself say exactly how rapidly or vigorously a metal will react.