Radiation is produced by the disintegration of unstable atoms. The atoms of different elements vary by weight, determined by the number of protons and neutrons in each nucleus. Each element has a unique number of protons, which never changes, determining its “atomic number” and its position in the periodic table: hydrogen never has more than one proton; oxygen always has eight; gold has seventy-nine. But atoms of the same element may have varying numbers of neutrons, resulting in different isotopes, ranging anywhere from deuterium (hydrogen with one neutron instead of two) to uranium 235
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