Alkaline earth metals

Where the name comes from

Early chemists called substances that did not melt or change in fire “earths”. Lime, magnesia and baryta were earths that dissolved slightly in water to give alkaline solutions, so they were called alkaline earths. For a long time they were thought to be elements themselves. In 1808 Humphry Davy used electrolysis to show that each contained a metal, and the name passed from the oxides to the metals.

Two electrons, a little less reactive

Each alkaline earth metal has two electrons in its outer shell and loses both to form 2+ ions. Removing two electrons takes more energy than removing one, so these metals are less reactive than their neighbours in group 1. Beryllium barely reacts with water at all, magnesium reacts with steam, and calcium, strontium and barium react with cold water at increasing speed.

They are also harder and denser than the alkali metals, with higher melting points, because two electrons per atom take part in metallic bonding.

Colours in the flame

Heat their compounds and several of these metals give off bright colours, the basis of the flame test used in school laboratories and of fireworks.

• Calcium: brick red
• Strontium: crimson, the classic red of fireworks and flares
• Barium: apple green
• Magnesium: an intense white light when the metal itself burns

Essential to life and building

Calcium is the most abundant metal in the human body. It makes up bones and teeth as calcium phosphate, and calcium ions control muscle contraction and the heartbeat. Magnesium sits at the heart of chlorophyll, the molecule plants use to capture sunlight.

The same chemistry shapes the landscape. Limestone, chalk and marble are calcium carbonate, much of it formed from the shells of ancient sea creatures. Heated, limestone gives quicklime, the starting point for mortar and cement.

The dangerous members

Beryllium and its compounds are toxic; breathing in the dust can cause a chronic lung disease. Soluble barium compounds are poisonous too, although barium sulfate is so insoluble that patients drink it safely before X-ray scans. Radium, at the bottom of the group, is intensely radioactive and was once painted on watch dials until the harm to the workers became clear.