Polarity

When we talk about a covalent bond we also talk about polarity. //So what is polarity?//

As said earlier, in a covalent bond the two atoms share a pair of electrons. But the two atoms can be having different electronegativities which means that they can attract the electron pair to different extents.

For example chlorine has a higher electronegativity than hydrogen. Therefore in hydrochloric acid (HCl) the chlorine will attract the electron pair more strongly than the hydrogen. As a result the electron pair isn’t shared equally between the two; the electron pair is closer to the chlorine atom’s nucleus and further away from the hydrogen atom’s nucleus.  As the electrons are closer to chlorine there will be a //slight negative charge// on //chlorine// and since the electrons are further away from hydrogen there will be a //slight positive charge// on // hydrogen //.

We call this molecule (HCl) a polar molecule because it has two poles, one with a slight positive charge and one with a slight negative charge.

**//In general terms, the more electronegative element attracts the electron pairs more strongly, so the electron pair are closer to the more electronegative element and further away from the less electronegative one. As a result the more electronegative element acquires a partial negative charge while the less electronegative one acquires a partial positive charge. //**

However if the atoms in the molecule are identical then the molecule is not polar. Eg. Hydrogen molecule H 2 

**//The two molecules have the same electronegativity values so the electron pair is shared equally between them. The molecule is not polar. //**

In case of HCl above if we draw an arrow pointing from the partial positive part to the partial negative part, we have got what is known as the molecular dipole (a dipole comes from the fact that the molecule has a + and a – end).

However we can have a molecule which has atoms with different electronegativity values which isn’t polar.

An example in carbon dioxide. Oxygen has a higher electronegativity than carbon //(you can find out the eletronegativity values of different elements from page 8 of the data booklet).//

Therefore the oxygen atoms will attract the shared electron pair more than carbon resulting in the oxygen atoms gaining a partial negative charge and the carbon atoms gaining a partial positive charge. 

The arrows show the direction of the dipole, form positive to negative. In the case of CO 2 these dipoles cancel out so that there is no resultant dipole. Or what is known as the dipole moment. Therefore CO 2 is not polar.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">To find the dipole moment of a molecule, simply add the individual dipoles, if they cancel, there is no dipole moment which means that the molecule isn’t polar. If there is a resultant dipole, the molecule is polar.

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Note: the greater the difference in electronegativity the greater the polarity of the bond. //

By the end of this lesson you should be able to:
 * Predict whether a particular covalent bond will be polar.
 * Use bond polarities and the molecule's shape to determine whether the molecule is polar.
 * media type="googleplusone" key="" width="360" height="18" ||
 * media type="facebooklike" key="http%3A%2F%2Fibchem4u.wikispaces.com%2FPolarity" width="360" height="74" ||