Sodium chloride

Sodium chloride is the inorganic compound most present in human experience: it is found in the kitchen, in blood at 9 g/L (the origin of saline solution), in the oceans at 35 g/L and throughout food chains. Its face-centred cubic crystal structure — where each sodium ion is surrounded by six chloride ions and vice versa — has become the textbook example of ionic bonding. Crystal cohesion comes from Madelung electrostatic forces between opposite charges, strong enough to explain the high melting point (801 °C) despite a modest molar mass.

Historically, salt structured trade before currency: the word "salary" comes from the salt allowance paid to Roman soldiers, salt roads shaped medieval commercial geography, and the salt tax (gabelle) was one of the most contested taxes of the French Ancien Régime. Today, of the ~280 Mt mined annually, only about 6 % go to food. The majority (~60 %) feeds chlor-alkali electrolysis producing Cl₂, NaOH and H₂ — three pillars of modern inorganic chemistry. The remainder splits between road de-icing, industrial preservation (pulp, water treatment) and strategic stockpiles.

One notable chemistry anecdote: NaCl is extraordinarily hard to purify to electronics-grade levels. For semiconductor or pharmaceutical applications, multiple recrystallisations or ion chromatography are required. And the dissolution in water, paradoxically slightly endothermic (~3.9 kJ/mol), nicely illustrates the trade-off between lattice energy and ion solvation energy — a textbook thermodynamic case.