Dmitri Mendeleev's ability to predict the properties of germanium (Ge) before its discovery is attributed to his development of the Periodic Table of Elements. Mendeleev organized the known elements into a systematic table based on their atomic masses and chemical properties. His periodic table left gaps for undiscovered elements, and he made predictions about the properties of these missing elements based on the patterns and trends he observed among the known elements.
In the case of germanium, Mendeleev had identified a gap in his periodic table where an element with properties similar to silicon and tin should exist. He predicted that this unknown element, which he called "eka-silicon," would have a metalloid character, low reactivity, and four valence electrons, similar to silicon and tin. His predictions were remarkably accurate, and when germanium was discovered in 1886, it indeed exhibited these expected properties.
Mendeleev's success in predicting the properties of germanium provided strong evidence for the validity of his periodic table and its utility in organizing the elements based on their atomic structure and properties. This was a significant contribution to the field of chemistry and played a crucial role in our understanding of the elements.