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Bond polarity




AQA Content

use partial charges to show that a bond is polar
explain why some molecules with polar bonds do not have a permanent dipole.

Specification Notes

Electronegativity as the power of an atom to attract the pair of electrons in a covalent bond.
The electron distribution in a covalent bond between elements with different electronegativities will be unsymmetrical. This produces a polar covalent bond, and may cause a molecule to have a permanent dipole.


Understanding Electronegativity and Bond Polarity

Electronegativity is the power of an atom to attract the bonding pair of electrons in a covalent bond. The most electronegative element is fluorine. In general, the electronegativity of elements increases from left to right along a period, and up a group (ignoring noble gases).

Pauling Electronegativity Scale

Developed by Linus Pauling, this scale helps to assign values for bond polarity. A good rule is that if the difference between the electronegativities (EN) of two atoms is greater than 0.5, the bond is polar, that is...

∆EN > 0.5 = polar bond
∆EN < 0.5 = non-polar bond

Permanent Dipoles:
When the electronegativities are different, the electrons are pulled more to one end, leading to a permanent dipole across the covalent bond – a difference in charge between the two atoms.

A dipole moment is a vector quantity and is the sum of all the bond dipoles. It is measured in Debyes (D) and is drawn as an arrow, with a bar on the tail, with the arrow head pointing to the ∂– end.

Make sure you draw it from the ∂+ towards the ∂– part of a molecule.

Here's an example of how to draw the moment for HCl:
and for water:

Polar and Non-Polar Bonds
Non-Polar Covalent Bond:
Both atoms have equal electronegativity, and the electrons are held in the middle of the bond. An example is chlorine gas.

Polar Covalent Bond:
Hydrogen Fluoride (HF) is an example of a polar covalent bond where F has a higher electronegativity than H, and therefore has a stronger pull on the electrons. It has a permanent dipole.

Polar and Non-Polar Molecules
An entire molecule will be polar, overall, if it contains asymmetrical polar bonds.
The molecule will be non-polar if either:

It contains no polar bonds

It contains polar bonds but is symmetrical, so the polar bonds cancel out.

The 2D diagram is misleading - this as a tetrahedral strucure. The bonds are polar covalent:
Even though the bonds are polar, the moments do actually cancel out leaving the molecule non-polar.


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