Frederick Soddy observes spontaneous disintegration of radioactive elements into variants he calls "isotopes."
1902
Ernest Rutherford and Soddy publish theory of radioactive decay.
1903
Becquerel shares Nobel Prize for Physics with Pierre and Marie Curie for 1896 discovery of natural radioactivity.
1904
Rutherford discovers that alpha rays are heavy positively charged particles. In 1908, he is awarded a Nobel Prize in Chemistry for his work. J. J. Thomson proposes the "plum-pudding" model of the atom. In it the atom is envisioned as electrons surrounded by a soup of positive charge, like plums surrounded by pudding.
1905
Albert Einstein publishes the special theory of relativity regarding convertibility of matter and energy (E=mc2).