Systematic Chemical Formulas and the Four Families of Compounds

• Valency of Elements: Monovalent (H, Na, K, Ag, Cl, F, Br, I); Divalent (O, Mg, Ca, Zn, Ba, Pb); Trivalent (Al, Fe³⁺, N, P); Tetravalent (C, Si).
• Variable valency metals: Iron (Fe²⁺ Ferrous, Fe³⁺ Ferric); Copper (Cu⁺ Cuprous, Cu²⁺ Cupric).

• Polyatomic Atomic Groups (Radicals): Monovalent: Hydroxide OH⁻, Nitrate NO₃⁻, Nitrite NO₂⁻, Bicarbonate HCO₃⁻, Ammonium NH₄⁺ (the only positive radical).
• Divalent: Sulfate SO₄²⁻, Carbonate CO₃²⁻, Sulfite SO₃²⁻.
• Trivalent: Phosphate PO₄³⁻.

• Algorithm for Writing Formulas: (1) Write symbols side by side (Metal/Cation on left, Non-metal/Anion on right); (2) Write valencies beneath symbols; (3) Simplify ratios if possible; (4) Crisscross valencies.
• Example: Aluminum Sulfate -> Al (valency 3), SO₄ (valency 2) -> Crisscross gives: Al₂(SO₄)₃.

Classification of Chemical Compounds:

• (1) Acids: Hydrogen-containing compounds that dissociate in water releasing hydrogen ions (H⁺ or hydronium H₃O⁺).
• Properties: Sour taste, turn litmus paper from blue to red, react with active metals releasing H₂ gas.
• Mineral acids: Hydrochloric HCl, Sulfuric H₂SO₄, Nitric HNO₃.

• (2) Alkalis / Bases: Compounds dissociating in water releasing hydroxide ions (OH⁻).
• Properties: Bitter taste, slippery soap-like feel, turn litmus paper from red to blue.
• Examples: Sodium hydroxide NaOH (Caustic soda), Potassium hydroxide KOH, Calcium hydroxide Ca(OH)₂ (Limewater).
• Note: 'All alkalis are bases, but not all bases are alkalis' (only water-soluble bases are alkalis).

• (3) Oxides: Binary compounds resulting from the combination of oxygen with another element.
• Basic Oxides: Metal + Oxygen (e.g., Na₂O, MgO, CuO).
• Acidic Oxides: Non-metal + Oxygen (e.g., CO₂, SO₂, SO₃).

• (4) Salts: Ionic compounds formed from positive metal cations (or NH₄⁺) bonded with negative non-metal anions (except O²⁻) or negative radicals.
• Water-soluble salts: NaCl, K₂SO₄, NaNO₃, Ca(HCO₃)₂.
• Insoluble salts (precipitates): Silver chloride AgCl (white), Lead iodide PbI₂ (yellow), Barium sulfate BaSO₄ (white), Calcium carbonate CaCO₃.