<span>Mendeleev left gaps in his periodic table because the properties of known elements predicted other, as-yet-undiscovered, elements in these locations. As Mendeleev organized his periodic table, he recognized that these gaps would be filled as future scientists identified new elements.</span>
Given a mole each for iron and magnesium, the number of atoms of each element is equal. Iron has a greater mass due to its greater molecular weight. The correct statement among the choices is D.
