Answer:
The two elements predicted by Mendeleev were gallium (Ga) and germanium (Ge). Mendeleev's periodic table was arranged based on increasing atomic mass and similar chemical properties. He left gaps in his table where he believed elements should exist but had not been discovered yet. Mendeleev made predictions about the properties of these undiscovered elements based on their positions in the table and the patterns of properties he observed among known elements.
1. **Gallium (Ga):** Mendeleev predicted the existence of an element he called "eka-aluminum" to fit beneath aluminum in the periodic table. He predicted its atomic mass and some of its properties. In 1875, Lecoq de Boisbaudran discovered gallium and found that its properties closely matched Mendeleev's predictions. Gallium's atomic mass was very close to Mendeleev's prediction, and its properties, such as melting point and density, were consistent with what Mendeleev had anticipated.
2. **Germanium (Ge):** Mendeleev also predicted an element "eka-silicon" to fit beneath silicon in his periodic table. He suggested that its properties would resemble those of silicon but with some differences. In 1886, Clemens Winkler discovered germanium, and it indeed had properties that aligned well with Mendeleev's predictions. Germanium's atomic mass and properties like density and conductivity were in line with what Mendeleev had foreseen.
In both cases, the discovery of gallium and germanium provided strong evidence for the validity of Mendeleev's periodic table and his method of arranging elements based on atomic mass and chemical properties. The fact that these elements were discovered later and their properties matched Mendeleev's predictions was a significant validation of his periodic law and the periodic table he had proposed. This success also bolstered the understanding that the periodic table was not just a mere coincidence but a reflection of the underlying patterns in the behavior of elements.