The Periodic Table
The periodic table is the organized array of the chemical elements in order of increasing atomic number — the number of protons in the nucleus — from hydrogen (1) to oganesson (118), with a recurring pattern of properties called the periodic law. Dmitri Mendeleev's table of 1869 arranged the 70 elements then known by atomic weight, left gaps for missing ones and famously foretold gallium, scandium and germanium. Henry Moseley's X-ray work of 1913 showed the true ordering principle was atomic number. IUPAC now maintains the table, its 18 groups and the standard atomic weights (latest revision 2024), and approves new names — nihonium, moscovium, tennessine and oganesson joined in 2016 to complete the seventh row. Today 118 elements are known, and researchers hunt for elements 119 and 120 and the long-predicted island of stability.