Marie Curie · Science
How radioactivity revealed that the atom is neither indivisible nor eternal, and pours out energy from a hidden store no one could explain.
For over two thousand years the atom had been, by definition, the smallest, simplest, most permanent thing - uncuttable, unchangeable, eternal. The whole of chemistry assumed atoms merely rearranged; they were never created, destroyed, or altered within. Radioactivity quietly detonated this assumption. If an atom spontaneously shoots out radiation, then something is happening inside it - it has parts, a structure, and a behaviour of its own. The supposedly uncuttable had been caught in the act of changing.
Working in Montreal and then with Frederick Soddy, Ernest Rutherford proposed a startling explanation: radioactive atoms are unstable, and each one spontaneously disintegrates, transforming into a different element and flinging out a particle and energy as it does. Radium decays toward lead, passing through other elements on the way. This was transmutation - the literal changing of one element into another - the ancient alchemical dream that orthodox chemistry had pronounced impossible. The atom was not eternal; it had a lifespan, and a death that made it something new.
If atoms transform spontaneously, when does any particular atom decay? Curie, Rutherford and Soddy uncovered a strange and beautiful answer. You cannot predict when a single atom will disintegrate - it may go in the next second or in ten thousand years, and nothing about its history, temperature, or chemistry changes the odds. Yet a population of such atoms obeys an exact law: in a fixed span of time, called the half-life, precisely half of them decay, then half of the remainder in the next such span, and so on, forever halving. Radium’s half-life is about 1,600 years; uranium’s is billions; some isotopes vanish in fractions of a second. Each radioactive substance has its own unchangeable half-life, a clock built into the atom that nothing in the laboratory can speed up or slow down.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned that radium gives off heat continuously without cooling or being fed. Explain why that was such a deep problem for nineteenth-century physics, in your own words.
Leads to Albert Einstein.
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