The Periodic Table
Mendeleev was brilliant but had problems - his periodic table had gaps and some elements seemed in wrong places when arranged by atomic mass. The modern table fixes this by using atomic number instead, creating perfect patterns.
Group 1 (alkali metals) have one outer electron, making them incredibly reactive - and reactivity increases down the group. They react violently with water, producing hydrogen gas and metal hydroxides. Group 7 (halogens) need one electron to complete their shells, but reactivity decreases down the group as atoms get larger.
Group 0 (noble gases) have full outer shells, making them almost completely unreactive - they exist as single atoms. Displacement reactions happen when a more reactive halogen kicks out a less reactive one from compounds.
Pattern spotting: Down Group 1, melting points decrease but reactivity increases. Down Group 7, it's the opposite - melting points increase but reactivity decreases!