Coulombs law violation??

PoincareConjecture
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Coulombs law violation??

Postby PoincareConjecture » Mon Sep 11, 2017 11:14 pm

So in the atomic Bohr model, Energy levels expand further and further out from the nucleus and one could use PE concepts ----Why would the energy increase the further away one gets from the nuclear force, it seems like a violation of Coulombs law----Can you help me understand, i can never seem to get a straight answer on this one, thank you
NS_Tutor_Andrew
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Re: Coulombs law violation??

Postby NS_Tutor_Andrew » Tue Sep 12, 2017 12:51 pm

Hi PoincareConjecture,

There are a few interesting things to note here! If I understand your question right, the short answer is "shielding" -- there's a good explanation here.

It sounds like there are a couple points worth clarifying here, though. Hydrogen atoms are often our model when we talk about electron excitation. They're super simple because they only have a single electron. That single electron will 'want' to be as close as possible to the nucleus, which you can understand in terms of electrostatic principles, so it takes energy to push it further away.

However, if you're dealing with an atom that has *many* electrons, you have to account for all of them. So, for example, the valence electron in Na will be in the 3s orbital, and that will be its lowest-energy state, because there's no "room" for it to get closer -- the closer valence shells are full.

It also might be worth clarifying that the nuclear force (strong or weak, although the strong one is the most relevant here) is a totally different force. There are four basic forces: gravitation, electromagnetic, strong nuclear, and weak nuclear (although I believe that electromagnetic and weak nuclear have been merged -- it's not relevant for the MCAT though). These are all completely different from each other. The electromagnetic force, with its magnitude expressed in terms of Coulomb's law, is as different from the strong nuclear force as it is from gravity.

That said, it can be useful to use the electromagnetic force to help explain why the strong nuclear force must exist. Have you ever wondered why the protons in a nucleus don't repel each other due to having the same charges and being concentrated in a tiny space? The answer is that on the scale of the nucleus, the strong nuclear force outweighs the repulsive electromagnetic interactions to "glue" the protons together. (The Wikipedia article is good background reading).

One final conceptual point: the fact that one force might "outweigh" another in a given context doesn't mean that the smaller force ceases to exist, or that a physical law was violated. Consider what happens when you use a magnet to attach something to a refrigerator. In that setup, the electromagnetic attractive forces between the magnet and the fridge 'outweigh' the force of gravity, but that doesn't mean that gravity stops existing or is violated, any more than using your muscles to lift something off the ground is a violation of gravity. This may seem obvious on the macroscopic scale, but sometimes it can be hard to apply those intuitions to very tiny things :).

Hope this helps!
Andrew D.
Content Manager, Next Step Test Prep.
PoincareConjecture
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Joined: Wed Mar 29, 2017 3:37 pm

Re: Coulombs law violation??

Postby PoincareConjecture » Wed Sep 13, 2017 9:45 pm

Man you ROCK!!! of course i never wondered why the protons in the nucleus didn't go flying off away from each other:)----The Hydrogen model was really helpful to conceptualize----But now you said in the Na atom the 3s Orbital is the lowest in energy?? Hmmmm obviously compared to electrons in 2s and 1s, but if you were to promote electrons from this other Orbitals the 1s would require the most energy to move to 3s...correct or would it be the 2s----i know the 1s would need to move along in steps so has to require the most energy due to its quantized state---has to first match up to 2s then move along to 3s....got it.....Thanks--i type a lot coz the concepts blend together in my head as i type away thanks Andrew i trulls appreciate you guys!!
PoincareConjecture
Posts: 93
Joined: Wed Mar 29, 2017 3:37 pm

Re: Coulombs law violation??

Postby PoincareConjecture » Wed Sep 13, 2017 11:10 pm

Quick recap i think there are two issues at hand after further review of the Libre online text an awesome resource by the way
1) Zeff the nuclear force an electron experiences is higher in S orbitals that have less shielding and lower in electrons located on Orbitals further out
2) Orbital energy levels are arrived at E=-Rh/n^2 calculus----and the further out you are from the nucleus the higher you are in energy level because the number becomes less and less negative approaching Zero----(Quantum mechanics stuff)....
Well thank you i am abit more comfortable now .....
NS_Tutor_Andrew
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Re: Coulombs law violation??

Postby NS_Tutor_Andrew » Fri Sep 15, 2017 11:30 am

You're very welcome! Glad to hear that explanation was helpful. In your most recent post, points (1) and (2) are correct, so I'm glad that got clarified! Re: the previous question about the 3s orbital in Na, two different things are true simultaneously: (1) the 3s orbital is higher-energy than the 1s/2s orbitals, and (2) the 3s orbital is the lowest-energy orbital that the valence electron can fit into because the 1 and 2 orbitals are full, if that makes sense.
Andrew D.
Content Manager, Next Step Test Prep.

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