Electrons and chemical bonds

[Ott Rovgeisha]

In a 2003 post about electrons and their behavior, which as a rule goes, tended not to get to a final solution, there was a quote by a poster:

"There is no chemical bond between chlorine and sodium. That is to say it is only electrostatic attraction, an ionic bond as opposed to a covalent bond. The pauli exclusion principle applies to electrons that occupy the same orbital, such as in a covalent bond."

I never participated in that converstaion but I found interesting this wording.

Would any of you be kind enough to discuss some questions, that arise from that.

1. What is the main and most important difference between a "chemical bond" and "electrostatic interaction"

I find it troubling, that people use those terms this way, implying that there is another fundamental attraction other than electronmagnetic, when it comes to molecular ponding. In fact many chemists believe there is something different, although they can not explain it.

2. How would you explain to people about a bond electron in an atom, who should not spin around nucleus, becuase it would lose its energy, having an orbital and spin?

3. As concpetually as possible, how would you describe, what exactly causes a "covalent bond"?
Where is energy before the bond making, how it redistributes and where does it end up?

4. It seems, that many people take the idea of "electron pair" into a rationalising path: evreyone has its own imagination and explanation as to what it is. How would you explain "and electron pair": how do the electron interact with the nucleus and each other, when they are "paired up"?

5. How would you describe, what is a bond? It seams to be a rather dangerous word, bcause it can lead to misconceptions. So what is a "bond" and more importantly, why isn't the attractions and repulsion in a NaCl solid called a "bond"?

Kind regards.

 

[Drakkith]

1. What is the main and most important difference between a "chemical bond" and "electrostatic interaction"

A chemical bond arises because of an electrostatic interaction. Per wiki: http://en.wikipedia.org/wiki/Chemical_bond

A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electrostatic force of attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction. The strength of chemical bonds varies considerably; there are "strong bonds" such as covalent or ionic bonds and "weak bonds" such as dipole–dipole interactions, the London dispersion force and hydrogen bonding.

2. How would you explain to people about a bond electron in an atom, who should not spin around nucleus, becuase it would lose its energy, having an orbital and spin?

Electrons certainly have orbital angular momentum, but they don't spiral into the nucleus because of the Rules of Quantum Mechanics.

3. As concpetually as possible, how would you describe, what exactly causes a "covalent bond"?
Where is energy before the bond making, how it redistributes and where does it end up?

I'm not quite sure what 'as conceptually as possible' means, but a covalent bond is a bond where two electrons occupy an orbital that surrounds both atoms involved in the bond and allows the paired electrons to interact strongly with both atoms. In effect, the two electrons are no longer in an atomic orbital, but a molecular orbital.

The energy was in the form of potential energy, and was released as heat, light, or kinetic energy after the bond.

4. It seems, that many people take the idea of "electron pair" into a rationalising path: evreyone has its own imagination and explanation as to what it is. How would you explain "and electron pair": how do the electron interact with the nucleus and each other, when they are "paired up"?

An electron pair is when two electrons occupy a single orbital, meaning they have the same quantum numbers except for their spin. This is pretty basic chemistry material. Even wiki says pretty much exactly what I said: http://en.wikipedia.org/wiki/Electron_pair

I can't say how the electrons interact with everything. I think the question is pretty vague to begin with.

5. How would you describe, what is a bond? It seams to be a rather dangerous word, bcause it can lead to misconceptions.

I don't see the problem. A chemical bond is defined pretty well in my opinion.

So what is a "bond" and more importantly, why isn't the attractions and repulsion in a NaCl solid called a "bond"?

A bond is exactly what I typed above. The attraction between Na and Cl in salt crystals is a bond. It's an ionic bond.

 

[Ott Rovgeisha]

A chemical bond arises because of an electrostatic interaction. Per wiki: http://en.wikipedia.org/wiki/Chemical_bond

A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electrostatic force of attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction. The strength of chemical bonds varies considerably; there are "strong bonds" such as covalent or ionic bonds and "weak bonds" such as dipole–dipole interactions, the London dispersion force and hydrogen bonding.
Electrons certainly have orbital angular momentum, but they don't spiral into the nucleus because of the Rules of Quantum Mechanics.
I'm not quite sure what 'as conceptually as possible' means, but a covalent bond is a bond where two electrons occupy an orbital that surrounds both atoms involved in the bond and allows the paired electrons to interact strongly with both atoms. In effect, the two electrons are no longer in an atomic orbital, but a molecular orbital.

The energy was in the form of potential energy, and was released as heat, light, or kinetic energy after the bond.
An electron pair is when two electrons occupy a single orbital, meaning they have the same quantum numbers except for their spin. This is pretty basic chemistry material. Even wiki says pretty much exactly what I said: http://en.wikipedia.org/wiki/Electron_pair

I can't say how the electrons interact with everything. I think the question is pretty vague to begin with.

Why do you link Wikipedia and copy-paste answers? I can read Wikipedia pretty well.. I assure you..

The questions were more like: how would you explain those things.

 

[Drakkith]

Why do you link Wikipedia and copy-paste answers? I can read Wikipedia pretty well.. I assure you..

Because they have the answers to your questions.

The questions were more like: how would you explain those things.

This is factual information, not the recipe list on Mrs. Riles cookie blog.

 

[Ott Rovgeisha]

Well, we can only be thankful that mr Richard Feynman did not do his lectures this way...

Anyway: interesting and meaningful answers/discussions are still of course be welcome :)

 

[Ott Rovgeisha]

Because they have the answers to your questions.
This is factual information, not the recipe list on Mrs. Riles cookie blog.

Hmmm.. Just in case: "We have rules in place to ensure Physics Forums is a fun, friendly, and productive environment."

Oh, just a thought that struck my students about the wikipedia entry:
"A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms"

"Yeah, and snow is snow, can't argue there, but the question was not about a raw definition, but conceptional differencies". (Just quoting my studends and colleagues).

Still: we would love to hear some ideas about the actual topic :) Thank you kindly in advance :)

 

[Drakkith]

"Yeah, and snow is snow, can't argue there, but the question was not about a raw definition, but conceptional differencies"

Again, I don't quite understand what 'conceptual differences' means. (Not surprising, as I have a language disorder)
For me science has always been about facts. There may be different analogies to help people visualize and understand something, but in the end it always seems to come down to a straightforward explanation or definition.

 

[DrDu]

Even in sodium chloride, the bond is not completely ionic but has a covalent contribution. In the most basic way, the mechanism behind a covalent bond is the following: According to Heisenberg's uncertainty relation $\Delta p \Delta x\ge \hbar$. Given that the kinetic energy of an electron is proportional to $(\Delta p)^2$, the energy of an electron will be lower when it has more space $\Delta x$ available. In a covalent bond it can move in the potential through of two atoms instead of one, hence its energy will be lower.

 

[Ott Rovgeisha]

Even in sodium chloride, the bond is not completely ionic but has a covalent contribution. In the most basic way, the mechanism behind a covalent bond is the following: According to Heisenberg's uncertainty relation $\Delta p \Delta x\ge \hbar$. Given that the kinetic energy of an electron is proportional to $(\Delta p)^2$, the energy of an electron will be lower when it has more space $\Delta x$ available. In a covalent bond it can move in the potential through of two atoms instead of one, hence its energy will be lower.

Thank you. From that, again, a question arises: if one says an electron has kinetic energy. So the first instincts is to state that this is due to its motion. So the question arises: what motion and relative to what?

Thank you!

 

[DrDu]

The total kinetic energy of a molecule can be separated into the kinetic energy due to the motion of the center of mass of the electrons and nuclei and the kinetic energy of the electrons and nuclei relative to the center of mass.
The kinetic energy of the center of mass motion is not quantized. Hence it is the motion of the electrons relative to the center of mass (which nearly coincides with the center of mass of the nuclei) relative to which we have to define momentum.

 

[cgk]

From that, again, a question arises: if one says an electron has kinetic energy. So the first instincts is to state that this is due to its motion. So the question arises: what motion and relative to what?

Quantum mechanics only deals in expectation values; that means that the concrete positions, velocities, etc. of an electron are not defined by this theory. You only get expectation values of positions, densities, momenta, etc. Now, in eigenstates these expectation values are stationary (that is, they do not change in time). But this does not necessarily mean that nothing is moving---you could conceptionally compare this, for example, with expectation values of time averages or ensemble averages of classical particles, which are also stationary even if things move around.

Thus, if you like to imagine happy little electron billiard balls flying around in a net of electronic cores, you can do that. There are even quantum-mechanical interpretations compatible with this picture (look up Bohmian mechanics). However, whether or not this picture corresponds to reality cannot be decided, as it is simply not part of the observable physical theory (which just given expectation values). Thus, most physicists would recommend to simply ignore this meta-physical reasoning and concentrate on quantities which can actually be physically observed and probed---everything which cannot be experimentally probed is not in the domain of physics. And, unfortunately, this includes the question on which/where/how the electrons move around or how best to imagine them.

 

[DrDu]

Quantum mechanics only deals in expectation values;

I wouldn't formulate it like this, or, at least give an alternative view: As in classical mechanics we have observables and velocity or momentum are among them.
So if we measure the velocity of an electron we will find a definite value. In physics, that's what we pay for: If we want to know whether something is moving, we have to measure it.
That the expectation value for e.g. velocity vanishes in a stationary bound state is also true in classical mechanics.