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Hi guys, I'm taking grade 12 chemistry right now and the teacher started with the introduction to Atomic Theory and its development. After thinking about all the stuff being taught, I am confused about certain concepts, and the teacher won't explain properly, hope someone can help me out.
The one big problem i have is that: we all know that the atom is made up of a nucleus and orbiting electrons. When an object is heated, it emits light. So according to my notes, it says that although electrons are orbiting around the nucleus and accelerating (change in direction because of its circular motion), and since its been scientifically proven that acceleration produce some type of light, the orbiting electrons would emit photons of electromagnetic radiation and lose energy, so theoretically they would crash into the nucleus.
So Bohr concluded that electrons have specific energy levels (electron shells), and if atoms absorb energy, the electrons will get excited and jump to a higher energy level, and when it returns to its "ground" position, the energy is released in the form of light.
The energy of a given energy level is determined by the following equation:
E = - R / n^2 ( where R is a constant of 2.18 x 10^-18 J, and "n" is the energy level/electron shell)
Here comes my problem, let's say I have a hydrogen atom (cuz its easy), its valence electrons are in n = 1 energy level. So if you plug it into the equation, you get a negative value. But the thing is, how can energy be negative?? or did i misinterpret the equation in some way?
ALso, it says that the energy of electron bound to the nucleus is lower than if the electron were at infinity, which imo makes sense. But next thing it says is that as n approaches infinity, Energy approaches ZERO. But how can that be? If n increases, doesn't energy also increase? so instead of the energy approaching zero, shouldn't it also approach infinity?
Hope you guys can help me out a little 
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ok well, although I do expect someone on here to answer those questions correctly, you should really just go after class and ask your teacher these questions...
I can answer the infinity thing, though. So you plug in infinity for n. You get R over infinity. Infinity is a really big number. When you divide a small number by a big number, you get a VERY SMALL number. Since infinity is VERY BIG, you get a VERY VERY VERY VERY small number. So, it is treated as zero.
edit: ok my explanation might not seem clear.
What is 1 / 1,000,000,000? it's like .000000000001 or whatever. Now imagine instead of 1,000,000,000, you have infinity. So now you should get the idea
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This is such a perfect question to ask in class though, doesn't make you look like an idiot, rather observant.
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On September 05 2008 07:12 blabber wrote:
ok well, although I do expect someone on here to answer those questions correctly, you should really just go after class and ask your teacher these questions...
I can answer the infinity thing, though. So you plug in infinity for n. You get R over infinity. Infinity is a really big number. When you divide a small number by a big number, you get a VERY SMALL number. Since infinity is VERY BIG, you get a VERY VERY VERY VERY small number. So, it is treated as zero.
Yeah i asked my teacher twice, and she isn't (imo) a qualified chemistry teacher. All she does in class is read the notes provided by some other teacher probably and doesnt teach us anything. She got a bit impatient after being asked the same question twice without a solid response. :/
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well if your teacher sucks, then you can always refer to your textbook... if you're still not getting it, that's when you should seek outside help
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On September 05 2008 07:12 blabber wrote:
ok well, although I do expect someone on here to answer those questions correctly, you should really just go after class and ask your teacher these questions...
I can answer the infinity thing, though. So you plug in infinity for n. You get R over infinity. Infinity is a really big number. When you divide a small number by a big number, you get a VERY SMALL number. Since infinity is VERY BIG, you get a VERY VERY VERY VERY small number. So, it is treated as zero.
edit: ok my explanation might not seem clear.
What is 1 / 1,000,000,000? it's like .000000000001 or whatever. Now imagine instead of 1,000,000,000, you have infinity. So now you should get the idea
But what about the negative in front??
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On September 05 2008 07:18 letsbefree wrote:Show nested quote +On September 05 2008 07:12 blabber wrote:
ok well, although I do expect someone on here to answer those questions correctly, you should really just go after class and ask your teacher these questions...
I can answer the infinity thing, though. So you plug in infinity for n. You get R over infinity. Infinity is a really big number. When you divide a small number by a big number, you get a VERY SMALL number. Since infinity is VERY BIG, you get a VERY VERY VERY VERY small number. So, it is treated as zero.
edit: ok my explanation might not seem clear.
What is 1 / 1,000,000,000? it's like .000000000001 or whatever. Now imagine instead of 1,000,000,000, you have infinity. So now you should get the idea But what about the negative in front??
That just means the number approaches zero from the negative side.
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I use to know the answer to this, but its been a few years since I have done chemistry. If I had my notes or my textbooks I could help you but they are at my parent's house. Sorry. Have you tried a quick google search?
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this is grade 12? more like grade 10.
first of all, electrons do not orbit the nucleus. that is the wrong way to visualize it. the electrons live in orbitals, and you can only determine the probability of finding the electron. you can never actually determine the definite point of a electron.
i kinda forgot, but im pretty sure the energy value is negative because thats saying the energy is released. think of it as change in energy. the change in energy is negative because energy is being released.
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wow thanks a bunch guys, really appreciate it =')
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Light is only emitted when the electrons change energy levels the amount of energy that is released is equal to your equation w/higher energy level plugged in minus (90% sure on this) the equation with the lower energy level.
dickless is right about everything.
You shouldn't trust me I can't get into chemistry at my retarded college even though I got a 5 on my AP test. /sarcasm /frustration
edit: for anyone who didn't know AP tests are test for college credit and the highest score is a 5.
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The negative sign means the electrons are "bound" to the atom.
and as n increases, the electrons are farther away from the atom, so it makes sense that the pull of the nucleus decreases.
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Basically, the Bohr model is wrong. Electrons do not orbit the nucleus like a planet does. The whole spinning into the nucleus thing was a problem that physicists had using the Bohr model because you have a positive charge in the center and electrons orbiting around it. That's all well and good, but the charges are opposite so there should be an attraction which slowly leads into the electron spiraling into the nucleus if what you detailed is true. This obviously does not happen.
As for "negative energy" that's a chemistry convention where if something is emitting energy, you put a negative sign in front of it. If it's absorbing energy it's positive. It's not actually "negative energy." Furthermore, the equation is not telling you the energy of the orbitals- it's giving you the energy of the transition between the orbitals. Big, big difference. It isn't that the energy approaches zero, the energy difference approaches zero. The emission spectra will show a series of lines. The lines start out fairly spaced out because the energy levels are pretty far apart to begin with, but then they start to converge in a mess of lines at some particular point and then end altogether.
The last line of this converging series of lines is the largest energy difference possible, which is n=infinity - n=1.
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Bill307
Canada9103 Posts
On September 05 2008 07:06 letsbefree wrote:
The energy of a given energy level is determined by the following equation:
E = - R / n^2 ( where R is a constant of 2.18 x 10^-18 J, and "n" is the energy level/electron shell)
Here comes my problem, let's say I have a hydrogen atom (cuz its easy), its valence electrons are in n = 1 energy level. So if you plug it into the equation, you get a negative value. But the thing is, how can energy be negative?? or did i misinterpret the equation in some way?
The energy in question is simply potential energy. Don't fret over having negative potential energy. Another example of negative potential energy is gravitational potential energy, given by the equation:
E = - G m1 m2 / r
Mathematically, it still works. E.g. as an object falls from the sky (i.e. as r decreases), its gravitational potential energy decreases. Likewise, as an electron's energy level n decreases, its potential energy also decreases.
As for why these potential energies are negative... well, I'm not entirely sure. But at least in the case of gravitational potential energy, I do know there is a lot of physics-related math that depends on it being negative. So I assume in both cases, the negative sign is there because of the math involved.
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On September 05 2008 07:59 Bill307 wrote:Show nested quote +On September 05 2008 07:06 letsbefree wrote:
The energy of a given energy level is determined by the following equation:
E = - R / n^2 ( where R is a constant of 2.18 x 10^-18 J, and "n" is the energy level/electron shell)
Here comes my problem, let's say I have a hydrogen atom (cuz its easy), its valence electrons are in n = 1 energy level. So if you plug it into the equation, you get a negative value. But the thing is, how can energy be negative?? or did i misinterpret the equation in some way? The energy in question is simply potential energy. Don't fret over having negative potential energy. Another example of negative potential energy is gravitational potential energy, given by the equation: E = - G m1 m2 / r Mathematically, it still works. E.g. as an object falls from the sky (i.e. as r decreases), its gravitational potential energy decreases. Likewise, as an electron's energy level n decreases, its potential energy also decreases. As for why these potential energies are negative... well, I'm not entirely sure. But at least in the case of gravitational potential energy, I do know there is a lot of physics-related math that depends on it being negative. So I assume in both cases, the negative sign is there because of the math involved.
We assign the value 0 to a certain level of magnitude arbitrarily. A value can be negative or positive depending on where you assigned the magnitude where it is 0. Just like height and potential energy. Sorry for my bad technical english 
OF course I could be wrong, but I think that's it.
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Or it's because the chemists claim dominion over the quantum world... mwhaha.
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Bill307
Canada9103 Posts
On September 05 2008 07:45 zer0das wrote:
As for "negative energy" that's a chemistry convention where if something is emitting energy, you put a negative sign in front of it. If it's absorbing energy it's positive. It's not actually "negative energy." Furthermore, the equation is not telling you the energy of the orbitals- it's giving you the energy of the transition between the orbitals. Big, big difference. It isn't that the energy approaches zero, the energy difference approaches zero. The emission spectra will show a series of lines. The lines start out fairly spaced out because the energy levels are pretty far apart to begin with, but then they start to converge in a mess of lines at some particular point and then end altogether.
This (specifically the part in bold) is incorrect: it is in fact a simplified form of the equation for the energy of an electron in an orbital around an atom. See here:
http://en.wikipedia.org/wiki/Energy_level#Orbital_state_energy_level
If you keep Z constant, then you can simplify the equation so that there's just one constant "R".
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Bill307
Canada9103 Posts
On September 05 2008 08:07 kemoryan wrote:Show nested quote +On September 05 2008 07:59 Bill307 wrote:On September 05 2008 07:06 letsbefree wrote:
The energy of a given energy level is determined by the following equation:
E = - R / n^2 ( where R is a constant of 2.18 x 10^-18 J, and "n" is the energy level/electron shell)
Here comes my problem, let's say I have a hydrogen atom (cuz its easy), its valence electrons are in n = 1 energy level. So if you plug it into the equation, you get a negative value. But the thing is, how can energy be negative?? or did i misinterpret the equation in some way? The energy in question is simply potential energy. Don't fret over having negative potential energy. Another example of negative potential energy is gravitational potential energy, given by the equation: E = - G m1 m2 / r Mathematically, it still works. E.g. as an object falls from the sky (i.e. as r decreases), its gravitational potential energy decreases. Likewise, as an electron's energy level n decreases, its potential energy also decreases. As for why these potential energies are negative... well, I'm not entirely sure. But at least in the case of gravitational potential energy, I do know there is a lot of physics-related math that depends on it being negative. So I assume in both cases, the negative sign is there because of the math involved. We assign the value 0 to a certain level of magnitude arbitrarily. A value can be negative or positive depending on where you assigned the magnitude where it is 0. Just like height and potential energy. Sorry for my bad technical english OF course I could be wrong, but I think that's it.
I think you're wrong. If you look at the page I linked above, since gravity is a conservative force, point #3 basically says the gravitational force must be the negative derivative of the gravitational potential energy, with respect to r. Therefore, the equation for the gravitational potential energy must have that form: we cannot arbitrarily move the 0 point anywhere.
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On September 05 2008 08:07 Bill307 wrote:Show nested quote +On September 05 2008 07:45 zer0das wrote:
As for "negative energy" that's a chemistry convention where if something is emitting energy, you put a negative sign in front of it. If it's absorbing energy it's positive. It's not actually "negative energy." Furthermore, the equation is not telling you the energy of the orbitals- it's giving you the energy of the transition between the orbitals. Big, big difference. It isn't that the energy approaches zero, the energy difference approaches zero. The emission spectra will show a series of lines. The lines start out fairly spaced out because the energy levels are pretty far apart to begin with, but then they start to converge in a mess of lines at some particular point and then end altogether. This (specifically the part in bold) is incorrect: it is in fact a simplified form of the equation for the energy of an electron in an orbital around an atom. See here: http://en.wikipedia.org/wiki/Energy_level#Orbital_state_energy_levelIf you keep Z constant, then you can simplify the equation so that there's just one constant "R".
So umm...can you help explain my questions?? I'm really confused nowwww
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EPT
