Why do atoms share electrons even though they repel each other?

[Freeman Dyson]

This is probably a stupid question but shouldn't electrons repel each other? So how can atoms share electrons? Can someone lay it out for me and tell me how close I am? When you bring atom close enough together, the like charges repel, but if you get them even closer the strong nuclear force overrides the repelling charge and brings them together? Am I close?

 

[diazona]

Electrons do repel each other. I'm not sure what that has to do with electrons being shared between atoms (a.k.a. covalent bonds), though... are you wondering how it is, if electrons repel, that atoms ever get close enough to form covalent bonds? If that's the question, remember that each atom has a positively charged nucleus as well. So the repulsion of the electrons is balanced out by the attraction of each electron to the other atom's nucleus. (Roughly speaking, that is... if you want to get into more detail it's a quantum mechanical thing)

The strong force has nothing to do with atoms bonding. It only applies inside protons and neutrons and other particles which are way smaller than atoms.

 

[Freeman Dyson]

Electrons do repel each other. I'm not sure what that has to do with electrons being shared between atoms (a.k.a. covalent bonds), though... are you wondering how it is, if electrons repel, that atoms ever get close enough to form covalent bonds? If that's the question, remember that each atom has a positively charged nucleus as well. So the repulsion of the electrons is balanced out by the attraction of each electron to the other atom's nucleus. (Roughly speaking, that is... if you want to get into more detail it's a quantum mechanical thing)

The strong force has nothing to do with atoms bonding. It only applies inside protons and neutrons and other particles which are way smaller than atoms.

Thanks. Yes, that is what I am asking. How do electrons from two different atoms come together to make bonds. How can any electrons "hang out" together?

The other point:

What I kind of mean is: Can you push atoms so close together that their nucleus' touch or almost touch and are brought together by the strong nuclear force? What I really want to know is the proper level of reduction. So matter attracts at one distance, at a shorter distance it repels, then at the ultimate short distance it attracts again. My rough belief was that atoms repel but if you can put them close enough together that nuclear forces can override this and pull them together.

 

[Lambda3]

Thanks. Yes, that is what I am asking. How do electrons from two different atoms come together to make bonds. How can any electrons "hang out" together?

The other point:

What I kind of mean is: Can you push atoms so close together that their nucleus' touch or almost touch and are brought together by the strong nuclear force? What I really want to know is the proper level of reduction. So matter attracts at one distance, at a shorter distance it repels, then at the ultimate short distance it attracts again. My rough belief was that atoms repel but if you can put them close enough together that nuclear forces can override this and pull them together.

You would first have to get rid of all the electrons before you attempted to do that, and then you can fuse the atoms together. However, it takes a lot of energy to do this, and I would think it would be really hard to fuse together two atoms with a large amount of nucleons in them. Nuclear fusion is the process that stars undergo in order to produce energy so that they don't collapse under their own gravity.

 

[diazona]

Thanks. Yes, that is what I am asking. How do electrons from two different atoms come together to make bonds. How can any electrons "hang out" together?

Okay, well... I'm not sure what to tell you except what I said in my last post, about the attraction to the opposing nucleus balancing out the mutual repulsion between electrons. I guess I could say that when you bring two atoms together, the electron orbitals change shape due to the presence of the two nuclei close to each other, and at certain distances the two-atom orbitals may have lower energy than the one-atom orbitals. But that's really just saying the same thing with fancier words.

Of course, not just any two atoms will attract each other and form a covalent bond. Some atoms, like helium or neon, will repel any other atom.

What I kind of mean is: Can you push atoms so close together that their nucleus' touch or almost touch and are brought together by the strong nuclear force?

Yes you can - it's called nuclear fusion This is the same process that allows stars to shine and that makes hydrogen bombs work. But it only works for atoms lighter than iron. For iron and heavier nuclei (that is, anything with an atomic number 26 or higher), the nuclei will be repelled no matter how close you bring them, because the nuclear binding energy per nucleon decreases once you get past atomic number 26. I'm not familiar with the full details of why that is, though.

It's important to note that nuclear fusion is a completely separate process from covalent bonding. By the time you bring two nuclei close enough to get them to fuse together, the electrons are completely out of the picture - the average distance between an electron and a nucleus is 1000-10000 times greater than the internuclear distance you'd need for fusion. Not to mention, if you were to provide enough energy to get two atomic nuclei that close together, the electrons would have so much energy that they wouldn't even be orbiting the atoms anymore; they'd be off somewhere flying around on their own.

What I really want to know is the proper level of reduction. So matter attracts at one distance, at a shorter distance it repels, then at the ultimate short distance it attracts again.

That's more or less right (at least for atoms lighter than iron). Bear in mind that at large distances, the attractive force is incomprehensibly tiny.

 

[Bob S]

First, the forces between two electrons, or between an electron and a nucleus, are electrostatic, which fall off as 1/r², and are not nuclear forces, which fall off exponentially. In mks units (which are inconvenient, but trustworthy), let’s write the electric field of a single electron with charge -e:

E = -e/(4 pi є₀ r²)

The work (energy) required to bring another electron from infinity to a separation r is

W = e²/(4 pi є₀ r)

Next, I will introduce a unitless fundamental constant in atomic physics, called the fine structure constant:
alpha = 1/137 = e²/(4 pi є₀ h_bar c)

Now, multiplying both numerator and denominator by alpha we get

W = h_bar c/(137 r)

Using the value of the fundamental constant h_bar = 6.58 x 10^-16 eV-sec and the speed of light c = 3 x 10⁸ meters/sec we get

W = 6.58 x10^-16 x 3 x 10²/(137 r) eV-meters/r = 1.44 x 10^-9 eV-meters/r.
Now using r = 0.53 x 10^-10 meters for the radius of the hydrogen atom, we get

W = 1.44 x 10^-9 eV-meters/ 0.53 x 10^-10 meters = 27.2 eV. (1 eV = 1 electron volt)

This is the attractive energy between a proton and electron spaced by 1 hydrogen-atom radius, or the repulsive energy of two electrons with the same spacing. (This is the depth of the electrostatic well for an electron bound to a proton, and is twice the binding energy of 13.6 eV).

The binding energy of one electron to a helium nucleus (helium positive ion) is about 4 times 13.6 eV, or 54.4 eV. An additional electron outside the helium positive ion “sees” +1e electric charge, because the full +2e charge of the helium nucleus is “screened” or hidden by the one orbital electron. So it will be attracted to and captured by the helium ion. In general, any positive ion of any element will attract and capture electrons.

 

[Freeman Dyson]

Thanks guys. Another question. PH. How is this related to charge? It is confusing. Does it simply mean that acid has a positive charge and base a negative?

:shy:

 

[diazona]

No relation at all. pH and charge are completely different concepts.

Basically pH is a measure of the concentration of extra hydrogen ions when you dissolve something in water. But of course all those hydrogen ions have opposing charges of some kind to balance them out, so the solution as a whole is always uncharged.

 

[nooma]

As stated above the pH is the relative concentration of hydronium ions in the solution. It is calculated using the formula pH = -log[H₃O⁺], where H₃O⁺ is the concentration of of the hydronium (or protons) in the solution in mols/litre. However although the concentration of positive ions increase as the pH decrease, the overall charge is neutral as the H₃O⁺ ions are dissociated from the negatively charged ions of the ionic compound they were bound with.

So all in all pH is not a measure of charge but the concentration of H₃O⁺ ions in the solution.