Final answer:
Mendeleev predicted the existence and properties of undiscovered elements using his periodic table, such as gallium and germanium, which confirmed the reliability of his periodic arrangement when they were later discovered.
Step-by-step explanation:
Dmitri Mendeleev, in developing his version of the periodic table in 1869, organized elements by increasing atomic mass and grouped them by their properties. Mendeleev was visionary in leaving intentional gaps for undiscovered elements and went so far as to predict the properties of these elements based on their position in the table. The most striking examples of his predictive power are the elements gallium and germanium, which he referred to as eka-aluminum and eka-silicon respectively, before they were discovered. Their properties closely matching Mendeleev's predictions greatly supported the acceptance of his periodic table.
Mendeleev was able to predict some things about unknown elements using his periodic table. He arranged the elements in the table based on their atomic masses and noticed that elements with similar properties tended to be grouped together. Based on this pattern, he predicted the existence and properties of then-unknown elements. For example, Mendeleev predicted the existence of eka-aluminum and eka-silicon, which were later discovered and found to have properties that closely matched his predictions. This successful prediction of unknown elements based on the periodicity of properties is one of the key contributions of Mendeleev's periodic table.