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Stable and Unstable Atomic Nuclei

Different kinds of atom are called elements, such as iron, oxygen and lead. Each element has different properties such as size and mass, and the ways it can bond to other elements.

Most atoms are stable and do not change from one element to another. An oxygen atom will normally stay an oxygen atom forever. In such an atom, the strong nuclear force holding the nucleus together is much stronger than the electric force pushing the protons away. The nucleons are held in place and cannot move very far.

The most stable kind of nucleus is iron. Nuclei larger than iron, like lead, or smaller than iron, like oxygen, are less stable. In these nuclei, the strong nuclear force and the electric force are more evenly balanced and the nucleus can stretch much more. Normally the strong nuclear force is still able to hold the nucleus together.

An unstable nucleus, like uranium, can stretch so far that the strong nuclear force is no longer strong enough to hold the nucleus together-and the electric force breaks the nucleus apart in a process called fission. The process of fission was first described by Otto Hahn and Lise Meitner.

The periodic table
magnifyThe periodic table

Lise Meitner
magnifyLisa Meitner
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