Absorbtion Spectrum: Atom Sublevels & Compounds

[scott_alexsk]

Hello,
Am I correct in thinking that the sublevels of an atom dictate its absorption spectrum? Also why is that a compound can absorb the same spectrum at every point, rather than individual atoms absorbing their respective spectrums?
Thanks,
-Scott

 

[photon79]

Am I correct in thinking that the sublevels of an atom dictate its absorption spectrum?
what do you mean by "sublevels dectate"? we get absorption spectrum when the atoms absorb some of the incident light..its because at these frequencies the electrons in the atom can be excited n reach higher states absorbing the radiation.

Also why is that a compound can absorb the same spectrum at every point, rather than individual atoms absorbing their respective spectrums?
A compound is formed when the orbitals of the individual atoms overlap, so there will be re-arrangement of electronic configuration when a compüound is formed. so it will absorb radiation depending on the new electronic energies rather than the individual atoms.

I hope it is clear from what I understood from your question.

 

[scott_alexsk]

Are you saying that atoms can absorb any frequency and they only generate specific frequencies in response to excitation by heat or other light?
-Scott

 

[photon79]

Are you saying that atoms can absorb any frequency
I didnt say it anywhere I think. These spectral lines are like finger prints of the atoms. Only certain frequencies can be absorbed and emitted, this from the fact that energy levels are quantized. The emitted or absorbed radiation is exactly the difference between the two energy levels involvedin the transition.

 

[scott_alexsk]

I thought you were saying something different last time, oh well. Alright so what allows for the transfer of this spectrum to all atoms within a compound. I mean it makes sense that the structure of the bonds will change the absorption spectrum, but what about the ones that are not directly bonded? I mean in certain complex molocuels like chlophyll, all points share the same absorption frequency, even though they are not bonded directly together.
-Scott

 

[photon79]

what allows for the transfer of this spectrum to all atoms within a compound.

I didnt get you saying transferring the spectrum,,if its a complex compound with many atoms, only some atoms will make bonds with some other depending on their orbital energies, so there are different groups of bonds corresponding to different regions of the spectrum.

 

[scott_alexsk]

Yeah orginally I thought that the distrubion would not be uniform, however when I was talking to someone about chlorpyll he said that because of resonance structure, it is uniform. I was wondering if anyone understood why this would be?
-Scott

 

[photon79]

I think he might be talking about the Nuclear Magnetic Resonence. I don't have idea about that,you must look into the absorption spectrum of clorophyl,,

 

[Gokul43201]

Hello,
Am I correct in thinking that the sublevels of an atom dictate its absorption spectrum? Also why is that a compound can absorb the same spectrum at every point, rather than individual atoms absorbing their respective spectrums?

When a compound does not retain the physical or chemical properties of the constituent elements, why do you expect it to possesses the spectral signatures of the atoms that make up the molecule ?

The constituent atoms lose much of their individuality in a molecule. The excitation/emission spectrum of a compound are related to the molecular energy levels of the molecules that make up the compound. The only spectrum that will show some characteristics of the induvidual atoms will be the core excitation (x-ray) spectrum.

 

[scott_alexsk]

Thanks for respoding Gukul. The reason I thought that is because in single atoms the excitation spectrum is determined by the difference in energy levels within the atom. It was described to me before that as a result of resonence and the merging of orbitals in bonding, a compound retains a uniform absorption spectrum even though its constituent atoms differ. Can you tell me what is wrong with this view?
-Scott

 

[Gokul43201]

Thanks for respoding Gukul. The reason I thought that is because in single atoms the excitation spectrum is determined by the difference in energy levels within the atom. It was described to me before that as a result of resonence and the merging of orbitals in bonding, a compound retains a uniform absorption spectrum even though its constituent atoms differ. Can you tell me what is wrong with this view?
-Scott

There's nothing wrong with it, though there are two problems in your understanding.

The individual atomic orbitals are no longer present; they are replaced by molecular orbitals formed by the superposition of the valence atomic orbitals. The valence energy levels are therefore those of the molecular orbitals.

Second, you must keep in mind that an absorption spectrum is a statistically generated plot, with data coming from a macroscopic number of molecules all over the sample. So, in that sense, it is meaningless to speak of a uniform spectrum (by which, I think you mean to say that the spectrum generated at one spot on the sample is identical to that generated from a different spot).

Describe to me, an experiment where you can generate different spectra to verify "uniformity" down to whatever level you desire, and then I'll try and explain why you see what you see.

 

[scott_alexsk]

I suppose for all practical purposes the distrubtion is uniform, but as a result of resonence it is also uniform at the microscopic level. Is there a way to determine the absorption spectrum of a compound without physical testing?
Thanks,
-Scott