Polarity of bonds

[Fmdvot]

Hello,

 

I was thinking about polarity today and i started thinking about the major factors like electro-negativity of individual atoms, reactions of functional groups and acids, electric fields around an atom..

 

I came to the realization that I don't REALLY know why any of those effect polarity. my question is if anyone has some interesting facts or detailed explanations about any of those factors? or point me to some links as to any links.

 

For instance how are differences in electro-negativities generated?

 

Thank you for your input.

[mississippichem]

Pauling Electronegativity is defined by the following equation:

 

[math] \chi_{\rm A} - \chi_{\rm B} = ({\rm eV})^{-1/2} \sqrt{E_{\rm d}({\rm AB}) - [E_{\rm d}({\rm AA}) + E_{\rm d}({\rm BB})]/2} [/math]

 

Here it is expressed as the difference in electronegativity between two bonded atoms. But what is the origin of this electronegativity difference? The answer lies in the concept of effective nuclear charge:

 

[math] Z_{\mathrm{eff}}= Z - s.\, [/math]

 

Where "Z" is the atomic number and "s "is the electronic shielding constant for an electron. (finding an s value can be cumbersome, sometimes they are listed in tables).

 

So when elements with different formal charges/oxidation states and different effective nuclear charges are bonded together. There must be unequal "sharing" of electrons. I put the word sharing in quotes because you shouldn't think of this as a simple back and forth sharing of a particle. It is more of a shift in the probability density of in the bonding molecular orbital wave functions.

So this unequal sharing, if unequal enough, will cause a slight overall charge difference between the two bonded atoms. Which actually serves to strengthen the bond as well (known as ionic contributions to a covalent bond). This is apparent in the Pauling E.N. equation above as he has included terms for an AA, BB, and AB bond, accounting for differing degrees of polarity or ionic forces.

 

 

The above diagram is nice but doesn't account for the differing sizes of the fluorine and hydrogen atom to scale.

 

Check wikipedia, their article on electronegativity is good, and is where I got my above equations.

Edited October 3, 2010 by mississippichem

[Tornado]

Think about electrons as a cloud instead of a tiny particle. We cannot know exactly where electrons are at any one time in an atom/bond however we know the areas that they are likely to be in. In polar bonds the electrons are not closer to one atom than the other there is merely more probability that at any one instance the electron will be found closer to the more electronegative atom. The reasoning behind this is due to multiple factors. Like with magnets opposite charges attract this is important in bond polarity. For example with the OH bond the oxygen is small but has a large positive nucleus relative to the size and because of the lack of electrons in the oxygen atom to shield those in the bond. The effects felt by the electrons in the OH bond are stronger than the effects felt by the same electrons from the hydrogen atoms. This means that the electrons are more likely to be found towards the oxygen atom than the hydrogen atom.