When a chemical reaction occurs, atoms rearrange to form new compounds, but no new atoms are created nor are any destroyed. this concept is called conservation of mass. mass conservation can be seen in a balanced chemical equation, where the numbers of each kind of atom are the same on both sides of the reaction arrow?

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.  

it's 3.2 on edge, i just took the test.

answer: gold foil experiment - he shot alpha particles at a gold foil

explanation: