Final answer:
Mendeleev predicted the reactivity of undiscovered elements by arranging elements based on atomic mass and comparing it with other chemical and physical properties. The periodic pattern that emerged from this arrangement allowed him to make predictions about the properties and reactivity of the undiscovered elements.
Step-by-step explanation:
Mendeleev arranged elements in a table based on their atomic masses. By comparing atomic mass with other chemical and physical properties, he was able to predict the reactivity of undiscovered elements. Mendeleev noticed that when he arranged elements in order of increasing atomic mass, elements with similar properties appeared at regular intervals. This periodic pattern allowed Mendeleev to predict the properties and reactivity of the undiscovered elements.