In the 1920s, a group of young women began showing symptoms of a horrific, but unknown, disease. Many developed mouth sores and lost weight. Some found their jaws extending into a cancerous mimicry of a pharaoh’s beard, tumors sprouting from their bones. Their blood cells and body tissues died, causing anemia and necrosis.

Known as the Radium Girls, these women covered watch faces with glowing paint made from the radioactive element, ultimately condemning them to painful disease and death. But the cause of their distress was not linked to radioactivity right away. The man who had discovered this new property of matter, Henri Becquerel, had simply called it “an unexpected phenomenon.”

In 1895, the French physicist realized that uranium salts applied to photographic plates somehow emitted particles that fogged the film. He sprinkled the uranium over a cross cut out of copper foil and saw the plate darken around the foil to produce a crosslike pattern. “I soon recognized that the emission was independent of any familiar source of excitation, such as light, electricity or heat,” wrote Becquerel.

Exactly what these particles were remained a mystery. Over the next decades, Nobel-Prize-winning work by Becquerel, Ernest Rutherford, and Marie and Pierre Curie revealed that understanding radioactivity meant fundamentally changing the world’s conception of atoms. The smallest and simplest components of elements, such as uranium and radium, were not indivisible units of matter.

First came the discovery of the electron in 1897 by English physicist J.J. Thomson. Electrons, he thought, were small negatively-charged particles that floated in a cloud of positive charge—the so-called “plum pudding” model of atomic structure.

Then, in 1911, his former student Ernest Rutherford found evidence for a new model—that the atom is composed of a dense center called a nucleus where the positive charge is concentrated and around which an array of negatively-charged electrons orbit.

By 1932 it was known that the nucleus had two components: positively-charged protons and uncharged neutrons packed densely together. The charged protons struggle to stay close as they simultaneously repel each other, like two magnets pointing their north ends towards one another. In some elements these strong, opposing forces weaken the stability of the nucleus, and the atom spits out particles until it reaches a more stable ratio of protons and neutrons. This is called radioactive decay.

Rutherford devised an experiment to characterize the particles that uranium atoms eject while they’re decaying. He placed a piece of uranium oxide in front of a device that detected electrical energy thrown off by the element. He then put successive sheets of aluminum foil between the two to see how many sheets were needed to dampen the uranium’s emissions.

He discovered that uranium emits two types of radioactive particles: alpha-particles, which are more readily blocked by the foil, and beta-particles, which are better at penetrating it.

In alpha decay, the unstable nucleus pops out a lump composed of two protons and two neutrons. That’s the alpha particle. The nucleus keeps emitting these until the element has reached a more comfortable, stable state.

Beta decay occurs when there are too many neutrons in the nucleus. This causes the atom to break a neutron down into a proton, which stays in the nucleus, and an electron—which becomes the beta particle shooting out from the atom. High-energy electromagnetic radiation, called gamma rays, can also be emitted in tandem with beta particles.

Alpha particles, beta particles, and gamma rays are all forms of ionizing radiation, a particularly dangerous form of radiation that can break chemical bonds, like the ones that hold molecules together in the human body. That’s what led to the cancers and anemia that the Radium Girls faced after chronic exposure to radium-tipped paint brushes.

Many lives were lost in the pursuit of radioactivity. Marie Curie succumbed to aplastic anemia brought on by exposure to radiation. Marguerite Perey discovered the radioactive element francium, and died of bone cancer. But in the years since these discoveries, grouped densely at the turn of the 20^th century, many useful functions have been discovered for radioactivity. Nuclear power plants split atoms to generate electricity. Doctors use radiation to kill rapidly dividing cancer cells. And archaeologists use radioactive decay to determine the age of remains.

Pierre Curie, who earned his Nobel Prize with his wife Marie for their work on the unexpected phenomenon, anticipated radioactivity’s potential for societal harm. “It can even be thought that radium could become very dangerous in criminal hands, and here the question can be raised whether mankind benefits from knowing the secrets of Nature,” he said in his Nobel speech. He concluded, though, that he hoped that, “mankind will derive more good than harm from the new discoveries.”