the.com/periodic table

means The chart that arranges all known chemical elements by atomic number and recurring properties, so you can predict how an element behaves just by where it sits.

from Credited chiefly to Dmitri Mendeleev in 1869, who arranged elements by atomic weight and noticed properties repeating in cycles — so reliably that he left gaps for elements not yet discovered and predicted their properties in advance, which were later confirmed. 'Periodic' refers to this repetition, or periodicity, of chemical properties at regular intervals as you move through the elements. The table was later reorganized around atomic number rather than weight once the structure of the atom was better understood, but the core insight — that arrangement reveals kinship — was Mendeleev's.