Final answer:
Mendeleev arranged the periodic table by atomic mass in rows and grouped elements with similar properties into columns. Over time, the table was reorganized by atomic number, enhancing its predictive power.
Step-by-step explanation:
Mendeleev arranged the elements into rows in order of increasing atomic mass so that elements with similar properties were in the same column, also known as a group. Therefore, the correct answer to the fill-in-the-blank is c) atomic mass / column. Mendeleev's early version of the periodic table was based on atomic mass, not atomic number, which is the basis for the modern periodic table. Over time, the periodic table evolved to arrange elements by increasing atomic number, recognizing that this was a better predictor of chemical properties than atomic mass.
Elements in the same group share similar properties and have the same number of valence electrons. The rows, or periods, on the periodic table represent elements with the same number of electron shells. While Mendeleev did have some elements out of order based on their atomic masses in order to keep elements with similar properties together, this irregularity was corrected once the table was reorganized according to atomic number.