Electronegativity and Polarity of Bonds
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When two identical atoms form a covalent bond, as in H2 or Cl2, each have equal share of the electron pair.
the electron density is the same on both ends of the bond, because the electrons are equally attracted to both nuclei.
But when different atoms bond, as in HCl, one nucleus attract the electrons from a bond more strongly than the other.
The electrons are shared, but unequally.
There are partial charges on both sides of the bond, indicated by a lowercase Greek letter delta, .
In HCl, the chlorine carries a partially negative charge (-) while the hydrogen carries a partially positive charge (+).
This is because the chlorine atom attracts the electrons stronger than the hydrogen.
A bond that carries a partial charge is called a polar covalent bond.
Because there are two poles of charge involve, the bond is a dipole.
If the entire molecule has a partial charge, it is a polar molecule.
This relative attraction of electrons in a bond is called electronegativity.
In HCl, the Cl is more electronegative than the H and attracts the electrons.
The electrons spend more time around the Cl than the H.
Looking it up on a periodic table or a similar table, one can see the H has an electronegativity of 2.1 while Cl has one of 2.9.
Therefore, the electronegativity difference is 0.8.
Electronegativity decreases as you go down a group and increases as you go left to right in a period.
The degree of the polarity of the bond is also called the ionic character of the bond.
The bond becomes more than 50% ionic when an electronegativity difference exceeds 1.7.