Can someone explain electronegativity, please.

[TylerH]

I'm taking AP Bio and we're covering basic chemistry needed for it. I've not had chemistry yet, so I'm left wondering about the interesting stuff, like electronegativity.

Basically, what I'm interested in, is: why is H₂O polar? What is it about O that makes it magnetically attract H?

Thanks,
Tyler

 

[Bloodthunder]

Electronegativity, symbol χ (the Greek letter chi), is a chemical property that describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself and thus the tendency to form negative ions.

Water is 2 hydrogen atoms covalently bonded to the oxygen atom, which in other words is just the sharing of a pair of electrons between them. However, the electronegativity of oxygen is greater than the electronegativity of hydrogen atom, so oxygen attracts the electrons closer towards itself, though not completely, resulting in a partial negative charge on the oxygen and partial positive charge on the hydrogen. So there is a dipole along the H-O bond, which makes it polar.

Also, O is not magnetically attracted to H. In covalent bonding, due to the sharing of electrons, while each atom wants to complete it's stable outer structure (8 electrons for O, 2 for Hydrogen), it is also "unwilling" to give up the electron completely due to the strong electrostatic attraction between the positive nucleus and the negative electron.

 

[ampakine]

Electronegativity is a measure of how much an atom or functional group attracts electrons.

 

[TylerH]

Water is 2 hydrogen atoms covalently bonded to the oxygen atom, which in other words is just the sharing of a pair of electrons between them. However, the electronegativity of oxygen is greater than the electronegativity of hydrogen atom, so oxygen attracts the electrons closer towards itself, though not completely, resulting in a partial negative charge on the oxygen and partial positive charge on the hydrogen. So there is a dipole along the H-O bond, which makes it polar.

So, since the O has more protons, it pulls the shared electrons closer to it and farther from H, increasing the negative charge of O and decreasing that of H?

Also, O is not magnetically attracted to H. In covalent bonding, due to the sharing of electrons, while each atom wants to complete it's stable outer structure (8 electrons for O, 2 for Hydrogen), it is also "unwilling" to give up the electron completely due to the strong electrostatic attraction between the positive nucleus and the negative electron.

Sorry, I was ambiguous with this one. I was referring to hydrogen bonds. How does electronegativity cause hydrogen bonds?

 

[Yanick]

So, since the O has more protons, it pulls the shared electrons closer to it and farther from H, increasing the negative charge of O and decreasing that of H?

It doesn't necessarily have to do with the number of protons in the nucleus. If that was the only factor that electronegativity would increase as you progress through the periodic table (increasing atomic number). This, however, is not the case. Flourine is the most electronegative atom, the farther you go from flourine on the periodic table the less electronegative the element. At this stage of your education its probably impossible to rationalize electronegativities (if it is in fact possible at all). Its something that I've not come across yet, and seems to just be a "take it for granted" kind of thing. The most important thing to learn is the trend of electronegativities on the periodic table.

Sorry, I was ambiguous with this one. I was referring to hydrogen bonds. How does electronegativity cause hydrogen bonds?

First of all you have to learn one more important concept that is fundamental to dipoles. Molecular geometry plays a huge role. Dipoles can be drawn as vectors along a bond, therefore they can add to or subtract from each other. Its very important to understand that water adopts a bent geometry. This bent geometry then leads to the 2 dipole vectors adding to each other and giving the overall molecule polarity. You can compare this with carbon dioxide for instance, which adopts a linear geometry. This linear geometry leads to the dipole vectors cancelling and no overall polarity to the carbon dioxide molecule (even though each bond is polar).

Hydrogen bonds are just a special/strong form of dipole-dipole interactions. These interactions occur in molecules with net partial charges (dipole vectors which do not sum to zero). In hydrogen bonds you need a hydrogen "sandwiched" between N, O, F. Dipole-dipole interactions will happen if there is a net dipole, regardless of the circumstances.

 

[TylerH]

Okay, that makes sense.

Thanks for the great explanations, everyone.