8.2 - 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.