Radioactive dating using uranium
Alpha decay is usually restricted to the heavier elements in the periodic table.
Nuclei can also decay by capturing one of the electrons that surround the nucleus.
After placing the uranium in an enclosed spot with no sunlight at all, Becquerel expected that the paper would be clear and free of dark spots.
Much to his surprise, the paper was darkened as if it had been exposed to the sun.
On a scale arranged according to the increasing mass of their nuclei, uranium is one of the heaviest of all the naturally-occurring elements (Hydrogen is the lightest). Like other elements, uranium occurs in several slightly differing forms known as 'isotopes'.
These isotopes differ from each other in the number of uncharged particles (neutrons) in the nucleus.