Final answer:
The precise predictions made by Mendeleev for the properties of unknown elements like eka-aluminum and eka-silicon, later identified as gallium and germanium, demonstrated the accuracy and predictive power of his periodic table.
Step-by-step explanation:
The close match between Mendeleev's predictions and the actual properties of newly discovered elements showed how accurate and predictive his periodic table could be. Dmitri Mendeleev, in arranging the elements, intentionally left gaps in his table, predicting the existence of then-unknown elements such as eka-aluminum and eka-silicon. The later discovery of gallium and germanium, which had properties that matched Mendeleev's predictions extraordinarily well, served as strong evidence of the reliability of Mendeleev's table. His method of organizing elements by increasing atomic mass, while sometimes placing certain elements out of strict mass order to maintain groupings by properties, highlighted the periodic nature of these properties and led to the periodic table being universally accepted.