Marie Curie, a giant in the field of physics and chemistry
After Henri Becquerel discovered radioactivity via studying uranium salts in 1896, Marie Curie took a strong interest in Becquerel's discovery, particularly the idea that, as gasses were passed through the path of the uranium's rays, they became conductors of electricity. This curious fact led to the inference that the atoms themselves might be spewing forth something from the inside, shattering the "unbreakable" stigma the atom had carried for literally thousands of years now. Curie thus discovered an element that was 400 times as active as uranium. She named this element polonium (after her native Poland), and then she found an element 900 times greater than that of uranium. This one she named, appropriately, radium. Marie was hence awarded the first ever female Nobel Prize, and the first person to be awarded a Nobel Prize in both chemistry and physics.
This "big idea" - that the atom wasn't just an immutable, undivideable piece of an element - was Curie's magnum opus, and one of the most eye-opening experimentally verified discoveries in the late 19th to early 20th century. Curie and her husband, Pierre, sacrificed their health to their study, too, as Pierre became sick due to radioactive poisoning before tragically being killed in a horse-drawn carriage accident. After this, Marie brought her primitive X-ray machine out into the field in World War I. Ultimately, she died of leukemia at the ripe age (considering) of 66.