Why are elements on the periodic table not arranged strictly by atomic mass?

Which group on the periodic table contains elements with the most diverse physical and chemical properties?

Which of the following pairs of elements are in the same period of the periodic table?

Which feature of Mendeleev's periodic table was considered particularly ingenious?

Which characteristic of an element determines its placement in the periodic table?

In the periodic table, what are the rows called?

Which feature of Mendeleev's periodic table made his theory about the organization of matter especially powerful?

Which term is used to describe the horizontal rows in the periodic table?