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What does electronegativity have to do with bond polarity?

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What does electronegativity have to do with bond polarity?

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Electronegativity is an element's attraction towards electrons in a covalent bond.  Polar bonds result from that uneven sharing of those electrons.

Think of it as a custody arrangement.  For an H-H bond, the atoms have the same electronegativity value so the electrons in the covalent bond are shared evenly (i.e. 50-50 custody).  For an H-F bond, F is more electronegative than H and pulls harder on those electrons so the electron density is centered closer to the F atom than the H atom.  (i.e. week/weekend split in custody).

F has the highest electronegativity and there is a periodic trend in these values.  As a result, the polarity of two bonds can be compared by looking at the differences in their electronegativity. 

  • H-F  difference in electronegativity = 1.9
  • O-F difference in electronegativity = 0.5

So, H-F is the more polar bond.  When you differences in electronegativity greater than ~2, you have ionic compounds where the electrons are being transferred, not shared as they are in covalent bonds.

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