This must me the isotope of an element with an atomic mass close to 69 u.
The only candidates are Zn and Ga.
Zn has a zinc-69 isotope with mass 68.926 u.
Ga has a gallium -69 isotope with mass 68.925 u.
The isotope is probably .
It has 30 protons and 39 neutrons.
For instance, when the nucleus of the isotope with a 68.926 amu, there are 30 protons and 39 neutrons in the nucleus. The best example for this is Zn (zinc-69 isotope) and Ga (gallium-69 isotope) which has 68.926 amu and 68.925 amu respectively.
the mass of the cathode increases because ions from the solution get reduced to atoms and become part of the cathode.
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