After he finished his work, Mendeleev predicted _____.
the atomic numbers of all the known elements
the discovery of the radioactive elements
the properties of three undiscovered elements
the properties of sodium, lithium, and potassium
To find the answer to this question, we can start by understanding who Mendeleev was and his contributions to the field of chemistry. Dmitri Mendeleev was a Russian chemist and inventor who is known for creating the periodic table of elements.
One of Mendeleev's notable achievements was his prediction of the properties of three undiscovered elements based on the patterns he observed in the periodic table. These elements turned out to be gallium, scandium, and germanium, which were discovered later and matched Mendeleev's predictions.
Therefore, the correct answer is: Mendeleev predicted the properties of three undiscovered elements.