The nucleus of mercury-180 contains 80 protons and 100 neutrons, and symmetrical fission would result in two nuclei of zirconium-90...

...They began with a beam of the highly unstable thallium-180, which has 81 protons and 99 neutrons, and which decayed primarily by the capture of an electron to convert one of the protons into a neutron, giving the 80 protons and 100 neutrons of mercury-180. This should then theoretically split symmetrically.

Instead, the mercury isotope split into ruthenium-100, with 44 protons and 56 neutrons, and krypton-80, with 36 protons and 44 neutrons. These are isotopes with incompletely filled energy levels.

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