Hard Chemistry Question

[thehstrybean]

OK, I have to write this super long Chemistry lab...the question we have to answer in our conclusion is this: why don't we see white light in the hydrogen spectrum? I'm confused as heck as to what the answer is? Can any genius' explain this?

 

[JustAnAverageGuy]

Seeing as how I have absolutely no clue what you're talking about, I googled and found this

The lightest elements, hydrogen and helium, give the simplest spectrum. Hydrogen produces a clearly discernible violet, blue-green, and violet bands. Helium produces violet, blue, blue-green, yellow, and red. Neon has several bands, mostly in the yellow-red end of the spectrum which is characteristic of the neon signs.

Every element will have a characteristic set of colored bands when viewed through a diffraction grating. These colors depend on the electronic structure of the element. The hydrogen spectrum is the simplest, because it has the simplest electronic structure.

We have to excite the elements to higher energy levels. The power supply does this. When the electrons "fall back" to a lower energy level, a photon is emitted that has a characteristic wavelength. We are only able to see the wavelengths in the visible spectrum. In general, there are many other spectral lines that we cannot see that are in the ultraviolet and infrared regions.

Source

I'm sure a more qualified member will assist you shortly

 

[thehstrybean]

I get that part, but to a chemistry Einstien, why don't we see white? If we see white somewhere, why not in Hydrogen? I guess what I'm trying to ask is, why doesn't Hydrogen give off white? Thanks averageguy, that was a help...I googled this question, but I got some off the wall, hard to understand answers...

 

[TuxDave]

Is that a trick question? White Light? There is no white light. White light is the composition of many light frequencies. The only way you can get white light is if hydrogen gave off a continuous spectrum of light.

 

[thehstrybean]

So then why do we see a continuous spectrum? Or do we?

 

[AFB]

Originally posted by: thehstrybean
So then why do we see a continuous spectrum? Or do we?

I thought that was an illusion

Edit: Guess not

 

[TuxDave]

Originally posted by: thehstrybean
So then why do we see a continuous spectrum? Or do we?

But who says it's all coming from hydrogen?

 

[simms]

Are you talking about the patterns that are given off when viewing H through a... dammit.. what are those things called..

Anyways, back to drinking!

 

[silverpig]

We see continuous spectra (ie white light) from thermal sources (ie blackbody radiation). The emitted photons get their energy from the kinetic energy of the atoms in the source. Basically the frequency of an emitted photon depends on the velocity of the particle that emitted it. Since a particle can have any velocity (below light speed of course), and there is no quantization, the emitted photons can have any frequency. Add up a whole ton of particles radiating this way and you'll get a continuous spectrum.

Hydrogen on the other hand radiates in a specific way. Hydrogen is a bound state of an electron and a proton and this system is governed by quantum mechanics (it's a good thing too... if it wasn't, all atomic matter in the universe would decay in about 10^-20 seconds or so). The atom radiates when an incoming photon knocks the electron into a higher energy level. This photon can have any energy and the atom will re-radiate this energy, provided that the photon isn't energetic enough to completely ionize the atom (blows the electron right off the proton). The electron then falls back to the lowest energy state in the atom, radiating energy as it goes. However, because of quantum mechanics, there are only certain discrete levels it can occupy. For example imagine you are climbing a ladder. You can be 1 rung, 2 rungs, 3 rungs from the ground, but because there is no 2.223rd rung, you can't be that far from the ground. The energy levels become pretty complex once you take into account all of the factors, but basically they follow a law like:

E(n) = (constants like c, hbar, 2pi, electron charge squared) x (1/nf^2 - 1/ni^2) where ni and nf are integers. You can see that completely ionizing a hydrogen atom (ni = 1, nf=infinity) requires 13.6 eV of energy. You can find the energy difference between any two levels using this formula. Of course it gets a little more complex, as each of these levels undergoes some splitting, but that's probably not required in your explanation.

It is because of this quantum mechanical nature of small things that hydrogen emits radiation only in discrete steps. This is where the good thing comes in. If the electron could radiate away it's energy continuously, it would be able to fall into the proton and atomic matter as we know it would not be possible.

edit: better analogy

 

[thehstrybean]

Now THAT got it...Thanks..you are an Einstein in my book...

 

[ChefJoe]

What lab were you doing ? This is a pretty basic P. Chem phenomenon. I'm working on my Ph. D. in Chemistry so I feel I should point out that your question uses a very questionable definition of "white" because while true white is supposed to be a mix of every visible portion of the spectrum and all of silverpig's response is correct in that reguard you've just proven that any monitor using colored pixels can't create "white" light which isn't quiet the case. Truth be known it's because the balance of the different colors it can make isn't quite right.