Discussion in 'Chemistry' started by pluto2, Jan 26, 2008.
In covalent bonds, what is attraction-to-repulsion stability?
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A coavelent bond is created by sharing electrons.
But why do atoms form bonds? If the atoms aren't ionized and feel no force between them (except the extremely weak gravitational force) then how do they form bonds?
Because it is mostly the effecient way to become stable. An atom ''needs'' other atoms to become a stable molecule. They do so by quantum entaglement by sharing electrons. If they are not ionized, they will find a subtable partner that does attract them, such as other attributes, such as spin.
Suffice to say however, ionization is nothing but a change in the isotope... Like a loss of an electron, which would make an imbalance between the measure of proton-electron ratio inside the atom.
What do you mean by stable?
Imagine two atoms with overlapping valence electrons like one of those Venn diagram circle things. If the center of each circle contains a positively charged nucleus, the electrons of each atom will be attracted to the nucleus of the other atom. If that attraction overcomes the repulsion of like charged particles it would be considered stable. I hope that is what you are asking and if anybody has anything to add or correct please feel free. I am a Chemistry student but I am no expert on the subject as of now so I may have overly simplified that and/or misinterpreted the question.
But shouldn't atoms with overlapping valence electrons repulse, not attract? Why are atoms with overlapping valence electrons attracted to the nucleus of the other atom instead of repelling?
The nucleus of one atom isn't attracted to the nucleaus of the other. The electrons are attracted to both nuclei, and both nuclei are attracted to the electrons. The electrons act like a "glue" that holds the nuclei together. The nuclie are trying to repell each other, but the force of their attraction to the shared electrons is stronger.
But why are the electrons attracted to both nuclei? Shouldn't the electrons of both atoms cause a repulsion between the atoms because like-charges repel?
Because all the electrons are negatively charged and both nuclei are positively charged.
There is some repulsion between the electrons, but their attraction to the nuclei wins.
Then shouldn't it be neutral (no attraction)?
Why does their attraction to both nuclei win? Does it depend on the Lennard-Jones potential?
The overall charge might be neutral, but there will still be areas with more or less positive and negative charge depending on exactly how the nuclei and electrons are arranged. Imagine you have a positive nucleus on the left and a negative electron on the right. They stick together because they have opposite charges, and together they are overall neutral. But if another electron comes in from the left, it will start to encounter the positive charge field of the nucleus before it encounters the negative field of the other electron (which is farther to the right). So even though over all out starting atom was neutral, there is still a region of positive charge on the left side that an electron can stick to.
It gets hard to describe this accurately without resorting to math, especially since the behavior of electrons around a nucleus and the formation of covalent bonds can only really be described with quantum physics, but that's sort of the idea behind it - even though the electrons are all repelling other electrons and nuclei are all repelling other nuclei, there are certain arrangements involving several nuclei and several electrons that allow for more interaction between opposite charges. I hope that makes sense.
But why is there a region of positive charge on the left side? If there is an electron to the left and a nucleus to the right shouldn't this region be neutral?
I said the nucleus was to the left and the electron was to the right. The area half-way between the nucleus and the electron (to the right of the nucleus, to the left of the electron) will be neutral. The region to the left of the nucleus will be positive, because the positive nucleus is closer than the negative electron. So if a second electron comes in from the left, it will see positive charge and be attracted to it.
Although in actual atoms it's a lot more complicated that "to the left or to the right" - I'm just trying to get the point across that it's possible for an additional electron to be attracted to an atom that's neutral overall because of the way the electrons are distributed.
I'm no chemist but I remember enough of my chemistry courses to recognize that there's another force at work here that has not been discussed. It has something to do with filling out each shell of electrons. Two in the inner shell, eight in the next two, then eighteen, etc.
The "noble" gasses have completely filled outer shells so there is no attraction between atoms. It's only atoms with incomplete outer shells that manifest this attraction. If an atom that has only seven electrons in its outer shell encounters another one that has only one, it might "borrow" it so that each atom now has a complete outer shell. (The next inner shell of the "lending" atom now becomes the outermost and it's already complete.) This is an ionic bond.
In other cases two atoms that each have only six electrons in their outer shell might each "share" two of theirs, resulting in both of them having a complete outer shell. This is a covalent bond.
There is a force at work that makes atoms "want" to have a complete outer shell.
That's still mainly just electrostatic attraction between the nucleus and the electrons. The whole thing about having a "filled outer shell" has to do with how well the electrons that are already around the nucleus shield any new, incoming electrons from the nucleus’s positive charge. Electrons don't shield other electrons in the same orbital very well, but they shield electrons in higher orbitals well. So you can usually keep adding electrons until a shell if full, since each new electron will still "see" a lot of positive charge from the nucleus. Once the shell is full, any new electrons will be much better shielded against the nucleus and won't see as much positive charge.
There's also something called "exchange energy," an electromagnetic effect that allows electrons to be slightly lower in energy when they are in an orbital with many other electrons whose spins are all pointed in the same direction, which is the main reason you sometimes get wacky ground state electron configurations for some of the transition elements. But like I said, mainly it's electrostatic attraction and repulsion.
Nice explanation. Thanks!
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