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.

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.

Ithink this is what you meant. the temperature at which a liquid begins to boil is called the boiling point. the boiling point of water is 212°f or 100°c.  and at that point will start to turn into steam (gas)