One of the greatest scientific achievements of the 19th century was Mendeleev’s discovery of the periodic table, which organizes the basic building blocks of matter into sets of substances with similar properties. These building blocks are chemical molecules that cannot be broken up into smaller molecules in short atoms.
Mendeleev’s table contained all the known elements in order of increasing relative atomic mass. Each row contained elements with the same joining power, and he swapped elements to suit this pattern. He left spaces for elements not discovered at the time.
Mendeleev made detailed predictions about the properties of some undiscovered elements, based on the gaps in his periodic table. There would be strong support for Mendeleev’s table of elements with these properties were found.