What interacted directly with the photographic plates to cause images in becquerel’s experiment?

uranium salt
radiation
unstable nuclei
radioisotopes

Answer is: uranium salt.

Antoine Henri Becquerel (1852 – 1908) was a French physicist and the first person to discover evidence of radioactivity.

Becquerel wrapped fluorescing crystal (uranium salt potassium uranyl sulfate) in a cloth, along with the photographic plate and a copper Maltese cross.

Several days later, he discovered that a image of the cross appeared on the plate.

The uranium salt was emitting radiation.

Because of this discovery, Becquerel won a Nobel Prize for Physics in 1903, which he shared with Marie Curie and Pierre Curie.

"the periodic table is arranged in order of atomic number."