Confusion about ionic bonding and variable valency

Encouraged · 30-01-2019 6:07pm #1

Before I ask the actual question I have here, I'd just like to say there is a certain amount of confusion with conceptualising how electrons revolve around atoms. I got through my biochem undergrad without really needing to understand this. The funny thing is that it seems that any rules for figuring out how compounds form (and what not) is based on what we learned in junior cert level anyway... that of (2, 8, 8, etc). But never the less, any time I try to think of new organic chemistry I learn in the context of Heisenberg's uncertainty principle (which perhaps I shouldn't), I get baffled. I asked somebody about all this once and he told me to look up "orbital penetration". I did and I got even more confused!

So if anybody knows of some tutorial video that tackles this issue (that only I seem to have), please let me know of it.

Now to my actual question; what I'm asking is why do the elements with a partially filled d sublevel gain/lose different amounts of electrons from what we'd expect? Merely saying that there's a small energy difference between 4s & 3d, does not properly explain why transition metal have variable valency. Not to mention the fact that iron doesn't even have a 4s energy level! One might even think that iron would prefer to gain 2 electrons to attain a complete 3d10 energy level.

And why is the 4s energy level lower than the 3d energy level. Is the 4s further away (distance) from the nucleus? I mean why else would it be denoted '4'?

It seems to me that any electrons in the 3d enery level would be considered to be in the 4 shell by beginner scientists. And that would seem to imply that there are (3d10, 4s2, 4p6, 4d10, 4f14) 42 electrons in the 4 shell!

If the answer to my question is a complicated one, let me know. I haven't thought of this in term of Heisenberg's uncertainty principle, because I'm afraid that would confuse me more!

Thanks

locum-motion · 31-01-2019 3:39pm

During my degree, I understood all this stuff.

Now after 22 years of practice as a Pharmacist, it's all just useless fluff at the back of my brain!

I wish I could help you, but I can't!! Sorry!

AlphabetCards · 06-02-2019 12:59pm

Look up shielding effects. Outer electrons (despite being in earlier orbitals) are shielded by the pull emanating from the nucleus.

https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Quantum_Mechanics/10%3A_Multi-electron_Atoms/Multi-Electron_Atoms/Penetration_and_Shielding

This should give you a decent idea of what's going on. Step away from the theoretical when approaching this, and picture it on a more physical/mechanical level.

Fourier · 09-02-2019 11:06am

AlphabetCards wrote:

This should give you a decent idea of what's going on. Step away from the theoretical when approaching this, and picture it on a more physical/mechanical level.

Just to note, in general Atoms cannot be understood mechanically.

Encouraged wrote: »

I'd just like to say there is a certain amount of confusion with conceptualising how electrons revolve around atoms

So first of all electrons don't actual orbit around atoms. I can go into this more if you wish.

Encouraged wrote:

Now to my actual question; what I'm asking is why do the elements with a partially filled d sublevel gain/lose different amounts of electrons from what we'd expect? Merely saying that there's a small energy difference between 4s & 3d, does not properly explain why transition metal have variable valency. Not to mention the fact that iron doesn't even have a 4s energy level! One might even think that iron would prefer to gain 2 electrons to attain a complete 3d10 energy level.

So let me start with one special case, it has nothing to do with orbit penetration as you have been told.

Encouraged wrote:

And why is the 4s energy level lower than the 3d energy level. Is the 4s further away (distance) from the nucleus? I mean why else would it be denoted '4'?

So the 4s energy level is actually higher than the 3d level, so as you said you'd expect 3d to be filled and then 4s. So placing an electron in 3d is less energy than putting one in the 4s shell.

However the 3d energy level is smaller in size than the 4s energy level. So although electrons will attempt to enter it first, if too many enter it then due to its small size you will have many negative charges in a small area. This causes them to repulse each other strongly and holding a bunch of charges together that are trying to move away from each other takes a lot of energy.

It turns out the energy it takes to stop them pushing each other out of the atom is more than the energy difference between 3d and 4s. So for a fraction of the energy the atom can just shove the electron up one level. Yes this is a higher energy level, but there will be no strong repulsion anymore.

On a very informal level the analogy would be putting people in a small night club at the bottom of a mountain (3d) or a big one at the top of the mountain (4s). Yes getting them up to the top takes a good deal of effort, but it's less than stopping the constant fighting in the small night club!

Epic Eir Epic · 19-02-2019 8:08pm

Fourier wrote: »

So first of all electrons don't actual orbit around atoms. I can go into this more if you wish.

Please do. What I've read about this made no sense with the pi sigma and something else type bonds seemed to suggest that the electrons are going through the nucleus.

Fourier · 19-02-2019 8:24pm

Epic Eir Epic wrote: »

Please do. What I've read about this made no sense with the pi sigma and something else type bonds seemed to suggest that the electrons are going through the nucleus.

Just to warn you in advance it's pretty odd. I'll answer in stages based on questions you might have.

Electrons don't have a position or a speed. When we arrange certain devices like powerful microscopes they'll report a position or a speed to the device by causing a part of the device to light up or moving a pointer on it.

The orbitals are actually just the shape of the region where you've a 95% chance to detect the electron. 5% of the time it'll be detected outside the orbital. However this isn't necessarily where the electron is, it's just where the device lit up or developed a mark.

Epic Eir Epic · 19-02-2019 11:14pm

Fourier wrote: »

Just to warn you in advance it's pretty odd. I'll answer in stages based on questions you might have.

Thank you, that's a good idea.

Fourier wrote: »

Electrons don't have a position or a speed. When we arrange certain devices like powerful microscopes they'll report a position or a speed to the device by causing a part of the device to light up or moving a pointer on it.

The orbitals are actually just the shape of the region where you've a 95% chance to detect the electron. 5% of the time it'll be detected outside the orbital. However this isn't necessarily where the electron is, it's just where the device lit up or developed a mark.

But an electron has to have a location at any given point in time? whether that location can be detected or not. Likewise with speed?

I think I recall something about being able to tell the speed and not the location and vis versa. So do you mean to say that that's now accepted as incorrect?

Fourier · 19-02-2019 11:22pm

Epic Eir Epic wrote: »

But an electron has to have a location at any given point in time? whether that location can be detected or not.

Believe it or not no, it doesn't have a location at any given point in time. It develops one only in response to probing with a microscope or similar. The position it develops is unpredictable, you can only workout the chances it'll develop a position at certain points, e.g. a 40% chance the position will be within a certain area.

Likewise with speed?

Same thing, it has no speed. Not that it's standing still as that's still a speed, just of 0 km/hr.

Note that you can't build a device that probes both its position and speed accurately, so it will possess one or the other.

Epic Eir Epic wrote: »

I think I recall something about being able to tell the speed and not the location and vis versa. So do you mean to say that that's now accepted as incorrect?

That's Heisenberg's Uncertainty Principle. It basically says the less unpredictable the position it will develop is, the more unpredictable the speed will be and vice versa.

Epic Eir Epic · 20-02-2019 1:46pm

Fourier wrote: »

Believe it or not no, it doesn't have a location at any given point in time. It develops one only in response to probing with a microscope or similar. The position it develops is unpredictable, you can only workout the chances it'll develop a position at certain points, e.g. a 40% chance the position will be within a certain area.

Note that you can't build a device that probes both its position and speed accurately, so it will possess one or the other.

That seems fundamentally impossible? So in other words no one really understands what's going on. We just have equations, and what not, that are
reliable.

I get how the microscope might interfere with the position that the electron would otherwise have, but would certain experts not disagree on that... that the electron would have to have a position even if it can't be detected?

Fourier · 20-02-2019 2:05pm

Epic Eir Epic wrote: »

I get how the microscope might interfere with the position that the electron would otherwise have, but would certain experts not disagree on that... that the electron would have to have a position even if it can't be detected?

There are results called the Kochen Specker theorem and Bell's theorem that show the electron doesn't have a position, i.e. it's not that you're interfering with a position that it would otherwise have. Both results have been tested to extreme precision and are among the most well tested results in science.

Not that it makes any sense to human intuition mind you!

Epic Eir Epic wrote:

That seems fundamentally impossible? So in other words no one really understands what's going on. We just have equations, and what not, that are
reliable.

Yes. Whatever the electron really is, it doesn't have a position or a speed or angular momentum or any of the quantities we use in our equations. We don't have an understanding of what the electron is really like, we just have equations for dealing with how it affects our equipment.

However there's a strong possibility that whatever it's really like can't be understood mathematically, which is the opinion of some experts (I can get you references if you like).

Leo Fatkar · 22-03-2019 12:27am

Fourier wrote: »

There are results called the Kochen Specker theorem and Bell's theorem that show the electron doesn't have a position, i.e. it's not that you're interfering with a position that it would otherwise have. Both results have been tested to extreme precision and are among the most well tested results in science.

That's sad to hear. If I were able to picture it somehow, I think it would help a great deal going forward.

Thanks anyway.

Confusion about ionic bonding and variable valency

Comments