Final answer:
Mendeleev did not use atomic number in his periodic table because the concept had not been discovered in 1869.
Instead, he arranged elements by atomic mass, leaving gaps for undiscovered elements, and corrected discrepancies by ordering some elements based on chemical properties rather than mass.
The Correct Option is; c) Atomic numbers were not known for all elements at the time.
Step-by-step explanation:
Dmitri Mendeleev did not use atomic number to arrange the elements on his periodic table because the concept of atomic number had not yet been discovered during his time. He published his periodic table in 1869, when only about two-thirds of the naturally occurring elements were known, and the scientists of that era had not yet realized that the atomic nucleus and the presence of protons determined the properties of elements.
The atomic number was later introduced by Henry Moseley, who in 1913 demonstrated through x-ray spectroscopy that elements should be arranged by atomic number rather than atomic mass, thus rectifying the issues with Mendeleev's arrangement.
Mendeleev had arranged the elements based on their atomic mass and left gaps for undiscovered elements, which allowed for predictions about these elements' properties and masses.
Although his table was predominantly accurate, there were a few exceptions where the arrangement did not perfectly match the properties of the elements, which he resolved by sometimes placing elements out of order of their mass to maintain consistent chemical properties. After the establishment of atomic numbers, the periodic table was reordered to rectify such discrepancies.