Which of the following element combinations will likely form polyatomic ions? cesium and chlorine oxygen and carbon sodium and bromine xenon and neon

Determine the wavelength of the light absorbed when an electron in a hydrogen atom makes a transition from an orbital in the n=3 level to an orbital in the n=7 level.

If anyone would be able to me out with these three questions it would be these are from the chem 2202 course.

If 2 molecules of one reactant combine with 3 molecules of another to produce 5 molecules of a product, then what is the representation of the reaction?