Who created the periodic table?
The first periodic table was developed by Dmitri Mendeleev in the mid-19th century.He wasn't the only person thinking along those lines ... both John Newlands and Lothar Meyer had proposed similar ideas
However, Newlands was largely criticized and ignored at the time, and Meyer didn't make any predictions, so Mendeleev's table (which did make predictions about the properties of several as-yet-undiscovered elements) is generally regarded as the first.
With the information he gathered about the elements, he was able to see that there were missing elements that hadn't been discovered. He could figure out the atomic masses of the missing elements by averaging the atomic masses of the elements above and below the missing one. One such element, which he called "eka-silicon" (eventually Ge) was missing, but with understanding of the patterns the periodic table made, he predicted the elements appearance, melting point, atomic mass, density, formula of oxide, and formula of chloride.
Throughout the years, other scientists were able to find or create these missing elements to form the table we know today. Today's periodic table is ordered by atomic number instead of atomic mass (as Mendeleev had started). Henry Moseley was the first to order the elements by atomic number so elements would fit together in groups/families and periods better in 1913.