2. Give one reason for why the periodic table is organized by atomic number and not by atomic mass?

Answer 1

The masses of a few elements would cause us to put them ahead of others. That means they would not fall into the proper groups/families if put in order of mass.

Answer 2

Atomic numbers are defined by the number of protons an atom has. (helium cannot have one proton; if it did, it would be hydrogen.)

The number of neutrons an atom has, however, does not change what type of element an atom is. Therefore, there can be atoms with more or less neutrons than the stable form (An atom with a different number of neutrons is called an isotope.). This would change the mass.

In short; the atomic mass can change, but the atomic number wont.

Answer 3

Mendelev origonally based his Periodic Table on atomic mass but then it had to be updated because the table was constructed based on similar properties of different elements. Some elements had to be adjusted.

Look at Iodine and Tellurium also Cobalt and Nickel.