The electronegativity of atoms can affect the polarity of covalent bonds.
Polar Covalent Bonds
If a covalent bond forms between atoms of different elements there will be a difference in the electronegativity of each atom. The atom that is more electronegative will attract the shared pair of electrons more.
Due to this attraction the electrons are (on average) closer to the more electronegative atom. This increases the electron density around the more electronegative atom and gives it a partially negative charge.
At the same time the electron density around the less electronegative atom reduces, giving it a partially negative charge (as electrons have a negative charge).
This means that there is a slight difference in charge across the bond. This difference in charge is called a permanent dipole.
Permanent Dipole – a difference in charge across a bond induced by a difference in the electronegativities of the bonded atoms.
Covalent bonds with a permanent dipole are called polar covalent bonds.
Polar Covalent Bond – a covalent bond with a permanent dipole.
Polar Covalent Bond Example
Hydrogen and Chlorine form a covalent bond to produce Hydrogen Chloride.
Chlorine is more electronegative than Hydrogen and so the shared pair of electrons are attracted towards the Chlorine atom (the red x’s represent the shared electrons):
This gives Chlorine a partially negative charge and Hydrogen a partially positive charge. The partial charges can be represented with a lowercase delta (𝛿) and a negative/positive sign:
This gives Hydrogen Chloride a permanent dipole and so it is a polar covalent bond.
Non-Polar Covalent Bonds
For covalent bonds in which both atoms are from the same element (such as a hydrogen molecule, H2) there is no difference in electronegativity between the atoms.
This means that there is no permanent dipole (difference in charge) across the bond.
The electrons are, on average, in the middle of the bond and so the bond is not polar.
Non-Polar Covalent Bond – a covalent bond in which the electrons are evenly distributed between both atoms involved in the bond.
Polar Molecules
As well as having a polar covalent bond, Hydrogen Chloride (HCl) is a polar molecule. This is the same for other simple molecules in which each atom only forms one polar covalent bond. This is because simple molecules like this are non-symmetrical.
However, not all molecules with polar bonds are polar molecules. Molecules that contain more atoms may be symmetrical and the dipoles of the bonds cancel each other out.
For example, tetrachloromethane (CCl4) contains 4 polar covalent bonds. However this molecule is symmetrical:
The dipoles are acting in different directions, so they effectively cancel each other out. Unlike HCl, there isn’t one side/end of the molecule that is slightly positive and one slightly negative. Therefore, the molecule is not polar but contains polar bonds.
Polar Molecule – a molecule with an overall dipole when considering all the bonds.
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