Final answer:
Mendeleev predicted that gallium would have a similarity to aluminum, and upon its discovery, gallium's properties matched these predictions, supporting the accuracy of his periodic table.
Step-by-step explanation:
Mendeleev's predictions on what the newly discovered element gallium would be like were amazingly accurate. He foresaw that gallium would share a similarity to aluminum, placing it in the table where he had predicted, called eka-aluminum. Gallium's properties matched the predictions so well that it reinforced the periodic system he designed. Mendeleev did not predict gallium to have nonmetallic properties, radioactive behavior, or to be in a gaseous state at room temperature.
These discoveries were an affirmation of Mendeleev's method of classifying elements and the accuracy of his periodic table—the elements are arranged in order of increasing atomic number in rows known as periods and columns known as groups. The elements in the same group of the periodic table generally have similar chemical properties. Furthermore, the elements can be classified as metals, metalloids, and nonmetals, which helped Mendeleev predict that gallium would be a metal, like aluminum, and not a nonmetal, metalloid, or a gas at room temperature.