Answer:
In Mendeleev's periodic table, elements were arranged according to their atomic masses.
Step-by-step explanation:
In Mendeleev's periodic table, elements were arranged according to their atomic masses. Mendeleev arranged the elements in such a way that elements with same physical and chemical properties lie in a vertical column of his periodic table. He discovered that physical and chemical properties of elements depends on their atomic masses.
In his periodic table there was some gaps. Mendeleev thought these gaps element are yet to discovered. Mendeleev was right because after some time some elements like aluminum and gallium were discovered and the properties of these elements was close to the properties which were predicted by Mendeleev.