First classification of chemical elements
The foundations of chemistry were laid by ancient Greek thinkers (Leucippus, Democritus, Epicurus) and by medieval scholars — al-Razi, al-Farabi, Beruni, Ibn Sina — who tried to group substances by their properties. About 30 elements were known at the end of the 18th century, 63 by the 1860s, and 118 today. Elements were first split by outward signs into metals and nonmetals: metals are shiny, conduct heat and electricity and are malleable (all solid except mercury); nonmetals lack these properties and are solid (S, C), liquid (Br₂) or gaseous (O₂, H₂, Cl₂). Hydroxides of typical metals are bases, those of nonmetals are acids. For amphoteric elements such as zinc and aluminium, the hydroxide reacts with both acids and alkalis: Zn(OH)₂ + 2HCl → ZnCl₂ + 2H₂O and Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O. This classification is not perfect: chromium forms CrO (basic), Cr₂O₃ (amphoteric) and CrO₃ (acidic).
“Metal or nonmetal?”: the teacher shows pictures or samples in closed containers (copper wire, sulfur powder, aluminium foil, charcoal); the class fills a properties table. Do not touch mercury or other chemicals; only the teacher handles samples.