Natural families of chemical elements
Elements with similar properties form natural families. Alkali metals (Li, Na, K, Rb, Cs, Fr) are soft, lighter than water (Li, Na, K), univalent in all compounds, and their hydroxides are soluble alkalis: 2Na + 2H₂O → 2NaOH + H₂↑. From lithium to caesium the melting point falls (Li ≈ 181 °C, Na ≈ 98 °C, K ≈ 64 °C, Cs ≈ 28 °C) while reactivity grows. Halogens (F, Cl, Br, I) form volatile HF, HCl, HBr, HI whose water solutions are acids; they are univalent in hydrides and salts and VII-valent in their highest oxides (except fluorine): H₂ + Cl₂ → 2HCl. Noble gases (He, Ne, Ar, Kr, Xe, Rn) consist of single atoms and hardly react. The pairs Mg–Ca, Al–B, C–Si and N–P also resemble each other. Alkali metals react violently with water, so such an experiment is shown only by the teacher with protection and in tiny amounts.
“Find the family”: cards show LiCl, KBr, NaF, HCl, He, Rb₂O ...; students sort them into the columns “alkali metal”, “halogen”, “noble gas”. No experiments with alkali metals are ever done at home.