Sequence Examine the periodic table. Which four pairs of elements would be
reversed in order if the elements were
listed by increasing atomic mass instead of
increasing atomic number?

Notice that tellurium is listed before iodine even though its atomic mass is higher. Mendeleev reversed the order because he knew that the properties of iodine were much more similar to those of fluorine (F), chlorine (Cl), and bromine (Br) than they were to oxygen (O), sulfur (S), and selenium (Se).

c. orbital shape i hope this is correct