Puzzle #1- Periodic Table Escape Room About 70 elements had been discovered by the mid-1800s, but until the work of the Russian chemist Dmitri Mendeleev, no one had found a way to relate the elements in a systematic, logical way. Mendeleev listed the
elements in columns in order of increasing atomic mass. He then arranged the columns so that the elements
with the most similar properties were side to side. He thus constructed the first periodic table, an arrangement
of the elements according to similarities in their properties. Mendeleev left blank spaces in the table because
there were no known elements with the appropriate properties and masses.

Mendeleev and others were able to predict the physical and chemical properties of the missing elements​.
Eventually these elements were discovered and were found to have properties similar to those predicted.
In 1913, Henry Moseley, a British physicist, determined the atomic number​ of the elements. Moseley
arrange the elements in a table by order of atomic number instead of atomic mass. That is the way the periodic
table is arranged today.