Calcium and magnesium
The main subgroup of group II is Be, Mg, Ca, Sr, Ba, Ra; Ca, Sr and Ba are called alkaline-earth metals. Mg is [Ne]3s² and Ca is [Ar]4s²: two outer s-electrons, oxidation state +2 in compounds, strong reducing agents but less active than the alkali metals. In nature they occur in compounds: limestone, chalk and marble CaCO₃, dolomite CaCO₃·MgCO₃, gypsum CaSO₄·2H₂O, phosphorite and apatite Ca₃(PO₄)₂, magnesite MgCO₃. They are produced by electrolysis of molten salts. Magnesium burns with a dazzling white flame (do not look at it): 2Mg + O₂ → 2MgO, 3Mg + N₂ → Mg₃N₂, 2Mg + CO₂ → 2MgO + C. Calcium reacts with water: Ca + 2H₂O → Ca(OH)₂ + H₂↑. CaO (quicklime) is made by roasting limestone: CaCO₃ → CaO + CO₂; it combines with water giving off heat: CaO + H₂O → Ca(OH)₂ (slaked lime). The saturated solution of Ca(OH)₂ is lime water (the reagent for CO₂) and its suspension in water is milk of lime. Chlorinated lime CaOCl₂ is used for bleaching and disinfection. Heating gypsum at 150–170 °C gives alabaster (CaSO₄)₂·H₂O, which sets quickly with water. Magnesium is part of chlorophyll, calcium phosphate Ca₃(PO₄)₂ is in bones and teeth; in Uzbekistan marble is quarried at Gazgan, Nurota and other deposits.
“Lime in construction”: on cards write the chain CaCO₃ → CaO → Ca(OH)₂ → CaCO₃ and put the condition over each arrow (heating, water, CO₂ from air). Explain the chemistry of lime mortar setting. Children do not handle lime or cement (they burn skin and eyes).