The periodic system of elements and the periodic law
On 1 March 1869 D.I. Mendeleev discovered the periodic law and at first tied it to atomic masses. Later Thomson discovered the electron (1897) and Moseley showed that the nuclear charge equals the atomic number (1913), so the modern wording is: the properties of elements and of their simple and compound substances depend periodically on the charge of the atomic nuclei. The period number equals the number of electron shells, and in main subgroups the group number equals the number of outer electrons (helium is an exception). The maximum number of electrons in a shell is 2n² (n is the shell number), and the subshells s, p, d, f are filled in the order 1s → 2s → 2p → 3s → 3p → 4s → 3d … Across a period from left to right the nuclear charge grows, the atomic radius shrinks, and ionisation energy and electron affinity increase, so metallic properties weaken and nonmetallic ones strengthen. Down a group the radius grows, outer electrons are held more weakly and metallic properties strengthen; the strongest metal is caesium and the strongest nonmetal is fluorine.
“Table relay”: split the class into two teams. Team members take turns placing the symbols of elements with Z = 3, 9, 12, 17, 19 on an empty periodic table on the board and saying each electron formula. The faster, more accurate team wins. No experiments: only chalk and a board.