Families 1-2 and 13-18 are called the... A. transition elements.
B. radioactive elements.
C. representative elements.
D. rare earth elements.

C. representative elements.

Explanation:

Families 1 - 2 and 13 - 18 are called representative elements or often times, main group elements.

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.