It has been a long time since I have studied science. I have forgotten some of the basics. And in some cases, it seems that I have forgotten to ask my professors some questions. So I hope this community can help. I have a series of questions, all relating to "frequency" of atoms/molecules. 1. When discussing electromagnetic radiation (especially the visible spectrum), we often refer to the wavelength (in units of nm). Since the speed of light is equal to the wavelength times the frequency, would we be wrong--or at least introduce the possibility of error--if we refered to the radiation by its corresponding frequency (in Hz) instead of wavelength? 2. Correct me if I a wrong: When we look at emission/absorption spectra, we are seeing the wavelength (or frequency, as per the above question) of electromagnetic waves which the atom(s) emit/absorb when its electron(s) change states. If this is true, then I have some related questions: (A) Since atoms can have a complex array of electrons (i.e., multiple electrons, and in multiple orbitals, with multiple possibilities for excited states), then some atoms will display multiple bands in the absorption/emission spectra. Moreover, the spectra of such an atom will not always be the same. For instance, if an atom has 2 electrons in the ground state, and one gets excited to the first-excited states, whereas the other gets excited to the second-excited state, then you can presume two bands will appear on the spectra. However, what if the second time you try this experiment, both electrons get excited to the first-state, or both to the second-state. Wouldn't you only see one band in each of these cases? (B) Can two elements absorb/emit the exact same wavelength, or is each wavelength reserved for a specific element? For instance, if an element absorbs/emits exactly 600 nm, does that mean that no other element can absorb/emit 600 nm? 3. Do emission/absorption spectra only occur for elements, or can they occur for compounds? For example, if the element nitrogen (a single atom) absorbs/emits 650 nm, will notrogen-gas (2 nitrogen atoms covalently bonded) have a unique absorption/emission spectra (i.e., something other than 650 nm)? I think that the answer here is that any compound will be broken down into its basic element before any absorption/emission occurs, and therefore only elements can absorb/emit EMR. 4. When it comes to the "frequencies" of molecules, my understanding is that any given molecule can vibrate at a variety of frequencies. This would be due to vibrational modes, such as symmetric, asymmetric, wagging, etc. These vibrations are measured with infrared spectroscopy. The units of measure are the inverse-centimeter. I have two questions about this: (A) Would it make sense to convert the inverse-centimeter into units of Hertz, or would that be nonsensical? For instance, can't I convert the inverse-cm into cm, then into nm, and then into Hz? (B) Do molecules have a fundamental frequency (that can be converted into Hz) at which they vibrate, or is there no such thing? I am referring to an overall vibrational frequency, rather than individual modes of vibration. Thanks!!