Research on radioactivity by Pierre (1859-1906) and Marie Curie (1867-1934) allowed the discovery of other radioactive elements like radium and polonium, which allowed scientists to begin understanding fundamental questions about matter.  After Pierre died in an accident, Marie Curie’s continued research on radioactivity that would open the door in understanding how elements change.   

The Periodic Table of the Elements changes as new elements were discovered.  In the 1930’s the table looked similar to the chart below.  Notice that the noble gases are on the left and not on the right as they are today.

Pierre and Marie Curie with
Becquerel (left) in their lab