Mendeleev arranged the table by atomic _____________ and Mosely arranged the table by atomic ____________

Mendeleev arranged the periodic table by atomic mass, while Moseley arranged it by atomic number.

Mendeleev's periodic table, created in the late 19th century, was organized according to the masses of the elements, which occasionally led to inconsistencies where elements with higher atomic masses were placed before those with lower atomic masses due to similar chemical properties.

However, Moseley's work in 1913 refined the classification of the elements by discovering that the atomic number, which counts the number of protons in an atom's nucleus, is a more accurate basis for organizing the elements. This adjustment resolved some of the discrepancies present in Mendeleev's table and laid the groundwork for the modern periodic table we use today, where elements are arranged based on increasing atomic number.

Thus, the complete answer is:

• Mendeleev arranged the table by atomic mass.
• Moseley arranged the table by atomic number.