What was Henry Moseley's major contribution to the periodic table and what year was it?
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Given: Henry Moseley and the periodic table
Russian chemist Dmitri Mendeleev was the first to organize the periodic table and predict elements that were not known but should exist. He did this in 1869. His periodic table was organized by relative atomic mass. There were some issues in organizing elements this way since there are a few elements that have lower atomic mass but they don't share the same group properties, so Mendeleev decided to put them in with the same group properties.
In 1913 Henry Moseley decided to organized the periodic table by atomic number. This solved the atomic mass issues.