21. For each of the following precipitation reactions, complete and balance the equation, indicating
clearly which product is the precipitate. If no reaction
would be expected, so indicate.
a. (NH4)2SO4(aq) + Ba(NO3)2(aq) →
b. HS(aq) + NiSO4(aq) →
c. FeCl3(aq) + NaOH(aq) →

i think it is d. e because the next one is igneous.

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.