How did becquerel’s work lead to the discovery of two new elements?

Becquerel discovered radioactivity by accident while studying a mineral containing uranium. He observed that with exposure to sunlight the mineral gave off a penetrating energy that exposed the film. While one day he tried the experiment one day the day was cloudy so he put it away in a dark dresser drawer and at the next day he found the picture of the mineral and later found out that the mineral did not need sunlight to expose radiation so he gave it to two young researchers that eventually isolated the two elements named polonium and radium.   

Sorry i dont understand the question can you rephase it?

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).