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How did the discovery of radioactivity show that atoms can change?

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Bohr model diagram of a hydrogen atom showing a green nucleus labeled H, a single electron orbit, and text with atomic mass 1.008 and electron configuration 1. — atom The Bohr model describes the structure of atoms, in this case hydrogen.

The discovery of radioactivity in the late 19th century fundamentally challenged the long-held scientific belief that atoms were indivisible and immutable. By observing that certain elements, such as uranium and thorium, spontaneously emitted penetrating radiation, scientists realized that the atom was not an inert, unchanging structure.12

Research by Ernest Rutherford and Frederick Soddy revealed that this radiation was associated with subatomic changes. Specifically, they discovered that radioactive atoms undergo transmutation, a process where one chemical element spontaneously transforms into another.123

This occurs through radioactive decay, such as:
* Alpha decay: The nucleus ejects an alpha particle, resulting in a daughter nucleus with two fewer protons and neutrons, thus becoming a different element.4
* Beta decay: A neutron in the nucleus converts into a proton and an electron, changing the atomic number and creating a new element.45

These findings proved that atoms are dynamic systems capable of internal change, ultimately leading to the development of quantum mechanics to explain phenomena that classical physics could not.1