the most common isotope of chromium has a mass number of 52. which notation represent a different isotope of chromium?

Not quite sure what your question is asking. Chromium-52, the most common isotope. Among lesser common isotopes there could be Chromium-53, Chromium-56, etc. I would have to look specifically at what exists. The percent abundance in the universe multiplied by each of these mass numbers and added determines the atomic mass number a displayed on the periodic table.