The periodic law of chemical elements
In 1869 D.I. Mendeleev arranged the elements in order of increasing atomic mass, noticed that properties repeat after a certain interval, and formulated the periodic law. Earlier, scientists such as Döbereiner (triads), de Chancourtois (cylindrical table), Newlands (octaves) and Meyer had also looked for patterns. Across the third period from Na to Cl, the highest-oxide valency rises from I to VII (Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, SO₃, Cl₂O₇), metallic character weakens and nonmetallic character grows, and the period ends with the noble gas argon; the next element, K, is again an alkali metal. Argon (Aᵣ ≈ 40) is heavier than potassium (Aᵣ ≈ 39), yet by properties argon must come first; this showed that properties depend on nuclear charge, not on atomic mass. The modern wording: the properties of elements and of the simple and compound substances they form depend periodically on the charge of the atomic nucleus. The atomic number equals the number of protons in the nucleus.
“Periodicity chain”: write H, He, Li ... Ar on cards numbered 1 to 18, lay them in rows of 8 and notice that the alkali metals fall in one column. Paper only.