Alkaline earth metals (group 2)

Two outer electrons

Each alkaline earth metal has two outer electrons in an s orbital, so its configuration ends in ns²: magnesium is [Ne] 3s² and calcium is [Ar] 4s². Losing both gives the arrangement of the previous noble gas and ions such as Mg²⁺ and Ca²⁺. Removing two electrons takes more energy than removing one, so these metals react less violently than the alkali metals.

The name comes from early chemistry: their oxides, once called earths, dissolve in water to give alkaline solutions, for example CaO + H₂O → Ca(OH)₂.

Physical properties

The alkaline earth metals are silvery-white, conduct electricity well and melt at much higher temperatures than the alkali metals: magnesium at 650 °C and beryllium at 1287 °C. Several of their compounds color flames, with calcium giving brick red, strontium crimson and barium pale green, which is why strontium and barium salts are used in fireworks.

Reactions and trends down the group

Reactivity increases down the group as the outer electrons get farther from the nucleus and easier to remove. Beryllium does not react with water, magnesium reacts only very slowly with cold water but burns in steam, and calcium reacts steadily with cold water: Ca + 2H₂O → Ca(OH)₂ + H₂. Strontium and barium react faster still.

Magnesium burns in air with a dazzling white flame: 2Mg + O₂ → 2MgO. Down the group the hydroxides become more soluble while the sulfates become less soluble, so barium sulfate hardly dissolves at all.

Uses of the alkaline earth metals

Calcium phosphate builds bones and teeth, and calcium carbonate forms chalk, limestone, marble and seashells; limestone is used to make cement. Dissolved calcium and magnesium ions cause hard water and limescale. Magnesium is a light metal used in alloys for cars, aircraft and laptops, and every chlorophyll molecule in green plants has a magnesium ion at its center.

Beryllium is light and stiff, so it is used in aerospace parts and formed the mirrors of the James Webb Space Telescope, but its dust is toxic. Barium sulfate is swallowed before X-rays of the gut and is safe because it is insoluble. Radium once made watch dials glow, and radium-223 now treats prostate cancer that has spread to the bones.