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Topic: Finding light's maximum wavelength (Atomic Structure)  (Read 2831 times)

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Offline Zolani13

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Finding light's maximum wavelength (Atomic Structure)
« on: March 25, 2012, 03:56:49 PM »
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The minimum energy required to remove an electron from the surface of lithium is 279KJ/mole. What is the maximum wavelength of light capable of doing this?

So this is an Atomic Structure question. For these types of questions, I would normally use the Bohr's model equation    :E:= -2.178x10-18(1/nf2 - 1/nf2) and substitute E with hv or hc/lambda, but of course that doesn't work in this case because the question doesn't give me any energy levels.

There's also the equation E=hc/lambda. I tried solving for lambda by plugging in E with 279KJ, but it doesn't give the proper answer, which is 427.7nm.

It asks for the maximum wavelength, but in order to use lambdamaxT = 2.898x10-3, I need the Temperature, which the question doesn't doesn't give me either!  >:(

So does anyone have any ideas of how I can find the maximum wavelength of the light? Even something to get me started would be appreciated.

Much thanks,
-Zolani

Offline JustinCh3m

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Re: Finding light's maximum wavelength (Atomic Structure)
« Reply #1 on: March 25, 2012, 04:08:39 PM »
to find the wavelength, you just need to find the E in the equation you've given.  If you're REMOVING the electron, do you need the 1/nf2 term?  Now, you just need to determine what ni is

Offline JustinCh3m

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Re: Finding light's maximum wavelength (Atomic Structure)
« Reply #2 on: March 25, 2012, 05:23:39 PM »
I should say that once you solve for E, you'll need to "convert" it to wavelength.  sorry, I was vague about that in my above post.

Offline blaisem

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Re: Finding light's maximum wavelength (Atomic Structure)
« Reply #3 on: March 25, 2012, 05:32:31 PM »
You plugged in E=279 kJ.  But you are dealing with the energy of removing one electron from the atom.  The context of your units is wrong.  The units of a number tell you what it is you are talking about.

In this case, consider: E=hf.  For E, is there a difference between plugging in E=279 kJ and plugging in E=279 kJ/mol?

In the first case, you are plugging in a flat amount of energy, 279 kJ.

In the second case, you are using the amount of energy necessary to remove a mol of electrons from Lithium.

These are the same amounts of energies, but there is an immediately apparent contextual significance.

Now is the question asking you for the minimum energy involved in removing a mole of electrons?

Also, the question asks for the complete removal of the electron, without mentioning anything about energy levels. You shouldn't need to worry about the Rydberg formula (Bohr model equation as you called it).  The question is simply asking for the light's relationship of wavelength to energy.
« Last Edit: March 25, 2012, 05:56:01 PM by blaisem »

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