Dmitri Mendeleev published his first widely recognized periodic table in 1869.
Mendeleev, a Russian chemist, arranged known elements primarily by increasing atomic weight and grouped elements with similar chemical properties. His table was influential because it left gaps for elements that had not yet been discovered. He used those gaps to predict properties of several future elements, including gallium and germanium.
The 1869 table was not identical to the modern periodic table. Scientists later learned that atomic number, rather than atomic weight, provides the correct fundamental ordering principle. Henry Moseley’s X-ray work in 1913 supplied strong evidence for that change.
Mendeleev is often called the father of the periodic table, but he built on earlier contributions by scientists such as John Newlands and Julius Lothar Meyer. The date 1869 refers to Mendeleev’s landmark publication, not the first attempt by anyone to classify elements.