Like most people, all my high school science classrooms had one wall papered with a huge, multi-coloured chart. It was, of course, the Periodic Table of the Elements.
Readers who were paying extra-close attention in their chemistry classes may remember that Dimitri Mendeleev came up with the periodic table in 1869. He organized the known chemical elements into a chart, based on their atomic masses and properties.
Mendeleev left gaps in the table for elements that hadn’t been discovered yet. The resulting table is a foundation of science that chemists still use, with some revisions, every day.