Because the product of (wavelength) times (frequency) is always the same number ... the wave speed. Since their product can't change, then obviously if one of them changes, the other one must change in the opposite direction.
Carbon-13 will have 6 protons (because it is carbon), 6 electrons, and 7 neutrons. Isotopes have a different number of neutrons.
This will take me a little bit let me research :)
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