Mendeleev left blanks in his periodic table (indicated by small dashes). Why, for example, did he put As, the next heaviest known element after Zn, in group 5 instead of ingroup 3 or 4?
Mendeleev had a theory that the elements would repeat in pattern similar to a musical scale. As was more like N than period 3 or 4
Rather than arranging the elements according mass Mendeleev arranged the elements according to properties. Mendeleev predicted that new elements would be discovered that would fill in the blanks he left in the periodic table.
As forms a compound
The predictions that Mendeleev made came true. These correct hypothesis gave evidence that Mendeleev's theory was correct. causing the periodic theory to become accepted.
Good theories make good predictions. Bad theories make bad predictions.