Main-Group Chemistry
Periodic trends, group reactivity, allotropes, hydrogen, and the key industrial processes.
Metallic character & ion charge
CORE RULEMetallic character rises down a group, falls left-to-right across a period. The group number gives the characteristic ion charge (group 1 → 1+, group 2 → 2+, group 17 → 1−).
Group reactivity
CORE RULEAlkali and alkaline-earth reactivity increases down the group (the valence electron is held more loosely). Group 1 is more reactive than group 2 in the same period.
Periodic trends for ranking
CORE RULEAtomic radius increases down a group, decreases across a period; ionization energy and electronegativity do the opposite (down ↓, across ↑).
Metals + water
CORE RULEGroup 1: 2M + 2H₂O → 2MOH + H₂ (all, cold water). Group 2 varies: Be none; Mg slow in hot water (steam → MgO + H₂); Ca, Sr, Ba: M + 2H₂O → M(OH)₂ + H₂. More vigorous for lower, larger metals.
Metals + oxygen / halogens
CORE RULEWith O₂: Li → Li₂O, Na → Na₂O₂ (peroxide), K/Rb/Cs → KO₂ (superoxide); group 2 → oxide MO (heavier ones, esp. Ba, form peroxide MO₂ when heated in excess O₂). With halogens → the ionic halide (2Na + Cl₂ → 2NaCl).
Allotropes
CORE RULEStructure drives properties: graphite (sp² layers → soft, conductive) vs diamond (sp³ network → hard, insulating); white P (strained P₄ → reactive) vs red P (polymeric → stable).
Hydrogen behavior
CORE RULEHydrogen is a reducing agent: +1 with a more electronegative nonmetal, −1 in ionic metal hydrides. H₂ reduces less-active metal oxides; an ionic hydride + water → metal hydroxide + H₂.
Preparing hydrogen
CORE RULEWater-gas (C + H₂O → CO + H₂), steam reforming of hydrocarbons, electrolysis of water (2H₂O → 2H₂ + O₂), and active metal + acid (Fe + 2H⁺ → Fe²⁺ + H₂).
Haber-Bosch (ammonia)
CORE RULEN₂ + 3H₂ ⇌ 2NH₃ (ΔH° ≈ −92 kJ), iron catalyst. Exothermic with fewer gas moles, so high pressure and low temperature favor NH₃ at equilibrium; a moderate temperature is a kinetic/yield compromise (speed over yield).
Ostwald (nitric acid)
CORE RULEOxidize ammonia in stages: 4NH₃ + 5O₂ → 4NO + 6H₂O; 2NO + O₂ → 2NO₂; 3NO₂ + H₂O → 2HNO₃ + NO.
Contact (sulfuric acid)
CORE RULE2SO₂ + O₂ ⇌ 2SO₃ over V₂O₅; absorb SO₃ into H₂SO₄ to make oleum, then dilute to H₂SO₄. Higher pressure and lower temperature favor SO₃.
Metallurgy
CORE RULEVery active metals come from electrolysis of a molten salt: Downs cell (2NaCl → 2Na + Cl₂), Hall-Héroult (alumina in molten cryolite → Al). Less-active metals come from chemical reduction.