Question

Q: Why do different substances show different light spectra?

I've tried researching for the answer, but I'm still not 100% on the answer. Can anyone give a clear and easily understandable explanation for this?

Answer 1

light spectra, as in absorption spectra?

Answer 2

As in the visible light spectra, I believe.

Answer 3

I'm not really sure. Do they enter a higher energy state..?

Answer 4

yeah, when something absorbs light, it's actually the electrons absorbing light, as they do this they enter an excited (of higher energy) and as they relax back to their original position (orbital) they emit light (fluoresce). Now because molecules/atoms have different spacings between these transitions they can only absorb certain wavelengths of light.

Answer 5

Okay, I sort of get the overall explanation - but you lost me at the part where you said different molecules have different spacings between the transitions.

Answer 6

okay, essentially the spacing between the orbitals which electrons transition (move to and from) is different for all molecules. Since this spacing is proportional to certain energy, and the frequency/wavelength of electromagnetic radiation dictates the energy of a photon, electrons can only absorb photons (light) of specific wavelengths.

Answer 7

Okay, I think I got this. So basically, different types of molecules contain different ionization energies (which measure the hold of the nucleus on the electrons). This means that a greater amount of energy is needed in order to transition the electrons from a higher orbital to a lower orbital, and thus they absorb more energy. In order to convert back to its original energy state, however, it releases the excess energy in the form of light. Therefore, the spectrum can be distinguished because the colour(s) of the light is determined by the amount of energy released.

Answer 8

(ie. red light is emitted when the electrons release a large amount of energy, and violet light when a small amount of energy is released)

Did I get that right? lol

Answer 9

no it's the other way around, red light is of lower energy (higher frequency but longer wavelength) than violet light.

Also, it's not ionization energy. Ionization implies that electrons are being removed from the substance, which is not the case.

Answer 10

Oh, right. But if it's not ionization energy, how does the attraction of the electrons to the nucleus relate to the emission line spectra?

Answer 11

the line emission spectra corresponds to the wavelengths of photons given off as the electrons relax from the excited state. The distance transited (the spacing between orbitals) by the electrons is proportional to the energy emitted.

Answer 12

Wait, so how do you know the distance/spacing between each orbital? Sorry! I'm just really confused by this topic. Usually I get it quicker than this.

Answer 13

no worries. i'm not sure i know the distance of these, i've studied the transitions for a hydrogen atom (a 1 electron system) it gets much more complicated after that because you're dealing with quantum mechanics. Take a loot at this http://www.kentchemistry.com/links/AtomicStructure/waveenergy.htm

Answer 14

Sorry but that website wasn't very helpful, the light spectra depends on the wavelengths and the wavelength is ineversly proportional to the photon's energy (amount of energy emitted) which you said is proportional to the spacing between the orbitals. But how do you know the spacing between the orbitals?

Answer 15

the spacing between orbitals is not something i know how to measure nor do i know how it's measured (i'm sure it can be estimated with computers in quantum mechanical simulations). These distances are in the order of femtometers (\(10^{-15}\) meters). What i'm saying is that the photons emitted/absorbed are of energy proportional to the distance between the transitions, an example of how this is done for the hydrogen atom is the rydberg formula \(\dfrac{1}{\lambda}=R_0(\dfrac{1}{n_f^2}-\dfrac{1}{n_i^2})\) which can tell use which levels (quantum number n) the transition was between.

Answer 16

I said that it is proportional to the distance between the transitions because energy is conserved.

Answer 17

"electrons can only absorb photons (light) of specific wavelengths" okay I think this is the problem, why? I'm sorry but I'm now back at square one, how would you explain "Why do different substances show different light spectra?" at a less complicated level

Answer 18

"Why do different substances show different light spectra?"

We know that light is quantized (comes in discrete packages e.g the photoelectric effect), electrons absorb photons of only certain energies in order for the transition to be "successful" (which increases the potential energy of the electron). If the photon isn't of enough energy (specific wavelength/frequency), the electron isn't able to make the transition and the photon is not absorbed, and the photon reflected is of the same energy as it came with (i.e. no energy was lost - because when a photon is absorbed some energy is lost, thermally, through vibrations, collisions). If the photon is of too much energy, the electron is ionized (removed from the atom).

"What makes the spacing between transitions different for molecules?"
Different elements have different distances between transitions because the nuclear "pull" (positive charge which is dependent on the amount of protons) experienced by the electrons is different. When atoms bond, they form different types (shapes/energies) of orbitals (called molecular orbitals). You can imagine that the molecular orbitals that arise from carbon monoxide will be different from carbon dioxide. So what makes them different? different combinations of atoms which range in the amount of protons in their nucleus and thus the excitation possibilities are different. (it's important to point out that radiation in the visible spectrum is only capable of exciting valence electrons... while radiation in the UV range will ionize valence electrons, x-rays will ionize core electrons (lowest energy) and cause a cascade of relaxations, radiation in the infrared red spectrum will make bonds vibrate, etc.).