An atomic nucleus usually follows its own rules: its decay doesn't care about temperature, pressure, or molecular neighbors. But the uranium-235m isotope is a striking exception. This nucleus sits in an excited state, like a tightly coiled spring, and sooner or later it releases the extra energy. Typically, such processes proceed at a constant rate, but here the chemical environment interferes with the nuclear clock.
In the new experiment, four uranyl compounds were compared: with fluorine, chlorine, bromine, and iodine. The half-life of uranium-235m turned out to be different: 25.32 minutes with fluorine, 26.05 with chlorine, 25.84 with bromine, and 25.44 with iodine. The stronger the halogen pulls electrons (that is, the higher its electronegativity), the longer the excited nucleus lives. Fluorine breaks the pattern: its half-life is the shortest because its bonding orbitals have the fewest 6p electrons, which participate in the internal conversion process.
This is the first direct observation of how molecular orbital formation changes nuclear decay. Previously, it was thought that the effect was limited to the simple electron density near the nucleus, but now it's clear that the structure of the chemical bond matters. This result paves the way for fine-tuning radioactive materials through their chemical environment — for example, in medicine or waste processing.
The half-life of uranium-235m in different compounds differs by only 44 seconds, but this difference is reliably measured and reveals a real influence of chemical bonding on the nucleus.