When Mendeleev arranged the elements, some did not fit the pattern. If he kept strictly to similar properties in each column, some elements would have landed in the wrong place. Instead of forcing them in, he left gaps in his table and said that these gaps were for elements that had not been discovered yet.
He then used the properties of the elements on either side of and above and below a gap to predict the properties of the missing element. He called one of them eka-silicon. He predicted that it would have a relative atomic mass of about 72 and form an oxide with the formula XO2.
Later, other chemists discovered the elements that fitted the gaps. Germanium was discovered in 1886 and its properties matched the predictions for eka-silicon very closely. Gallium, discovered in 1875, matched his prediction for another gap. Because the predictions came true, scientists gained confidence in the table and began to accept it.