Mendeleev's Prediction

Mendeleev could not make a complete table of the elements because many elements had not yet been discovered. He had to leave spaces in his table for those elements. For example, Mendeleev placed bromine (Br) in Group VII because bromine and chlorine (Cl) have similar properties. This placement left four spaces in row 4 between zinc (Zn) and bromine. Mendeleev had only two elements, arsenic and selenium, to fill those spaces, based on their masses. He placed arsenic and selenium in the columns where they fit best and left gaps in the columns labeled Groups III and IV.

Mendeleev was not the first to arrange elements in a periodic table. He was not even the first to leave spaces in a periodic table for missing elements. But he was able to offer the best explanation for how the properties of an element were related to its location in his table.

An excellent test for the correctness of a scientific model, such as Mendeleev's table, is whether the model can be used to make accurate predictions. Mendeleev was confident that the gaps in his table would be filled by new elements. He used the properties of elements located near the blank spaces in his table to predict properties for undiscovered elements. Some scientists didn't accept these predictions. Others used the predictions to help in their search for undiscovered elements.