Chemical Reactivity of Alkaline Earth Metals

Alkaline earth metals are also highly reactive, though as a rule less reactive than the alkali metal of the same period, since their higher ionization enthalpy makes them somewhat less ready to lose electrons. Beryllium, the smallest member, is a partial exception to several of the general-group trends described below, on account of its unusually small size and high charge density -- a pattern often described as its "anomalous behaviour," and one that also gives beryllium a diagonal relationship with aluminium (Group 13) in several of its properties.

Reaction with oxygen. Beryllium and magnesium are comparatively unreactive towards air at room temperature because each quickly forms a thin, continuous, strongly adherent protective oxide layer on its surface that passivates the metal underneath (magnesium ribbon, in particular, must be cleaned with sandpaper to remove this dulling layer before it will burn readily). Once ignited, however, magnesium burns in air with a characteristic brilliant white light to give a mixture of the oxide and the nitride:

Calcium, strontium and barium, being larger and more reactive, tarnish quickly and burn readily in air to give the oxide (and some nitride); the heavier members, calcium, strontium and barium, can also be made to form a peroxide on strongly heating the oxide in an excess of oxygen, in a limited parallel to the alkali-metal peroxide/superoxide pattern.

Reaction with water. Reactivity with water increases sharply down the group. Beryllium is the standout exception: it does not react with water even at red heat, because it develops an extremely thin, tenacious oxide film the moment it is exposed, which seals the metal beneath from any further attack -- exactly the passivating behaviour aluminium (its diagonal neighbour) also shows. Magnesium reacts only very slowly with cold water but readily and steadily with hot water or steam, evolving dihydrogen:

Calcium reacts moderately with cold water itself, evolving dihydrogen gas and forming calcium hydroxide, which is only sparingly soluble and so gives the familiar milky suspension called limewater:

Strontium and barium react with cold water still more vigorously than calcium.

Reaction with hydrogen. Calcium, strontium and barium combine directly with dihydrogen on heating to give ionic hydrides, $\text{MH}_2$, containing the discrete hydride ion $\text{H}^-$ (e.g. $\text{Ca} + \text{H}_2 \longrightarrow \text{CaH}_2$). Beryllium and magnesium, by contrast, do not form simple ionic hydrides under these conditions: $\text{BeH}_2$ and $\text{MgH}_2$ are covalent, polymeric solids, because the very small, highly charge-dense $\text{Be}^{2+}$ and (to a lesser extent) $\text{Mg}^{2+}$ ions polarize the electron cloud of the hydride ion strongly enough to give the bond substantial covalent character rather than a discrete ionic lattice. …