3.9. Dipoles. Polar and non-polar Molecules

Polar covalent bonds may cause dipoles to form

In the hydrogen chloride molecule, the bond between the hydrogen atom and the chlorine atom is polar.

• Cl is more electronegative than H
• Electrons spend more time with Cl than with H
• The molecule becomes a dipole: ^δ+H–Cl^δ–

Methane, CH₄, is not a dipole

The carbon and the hydrogen atoms bind to each other with electron pair bonds (covalent bonds).

• The electron pairs are as far from each other as possible.
• The electron pairs point into the corners of a tetrahedron (a triangular pyramid).

In a methane molecule, the hydrogen atoms end up in the corners of tetrahedron (triangular pyramid).

Ammonia, NH₃, is a dipole

The nitrogen and the hydrogen atoms bind to each other with polar electron pair bonds (covalent bonds).

• One electron pair is non-binding ⇒ The ammonia molecule is a trigonal pyramid.
• Nitrogen is more electronegative than hydrogen.
• The ”nitrogen end” is more negatively charged than the ”hydrogen end”.
  

The ammonia molecule is a trigonal pyramid with the nitrogen atom at the top.

Because of the polar covalent bonds, and the pyramidal structure, ammonia is a dipole.

Water, H₂O, is a dipole

Lewis structure and structural formula for water.

The oxygen and the hydrogen atoms bind to each other with polar electron pair bonds (covalent bonds).

• Two electron pairs are non-binding ⇒ The water molecule is bent.

Because of the two non-binding electron pairs (yellow) in a water molecule …

… the water molecule becomes bent.

Oxygen is more electronegative than hydrogen.

• The ”oxygen end” is more negatively charged than the ”hydrogen end”.
• The molecule becomes a dipole.

Because of the polar covalent bonds, and the bent structure, water is a dipole.

Carbon dioxide, CO₂, is not a dipole

Lewis structure of a carbon dioxide molecule.

Carbon dioxide structural formula.

Because of the two areas with high electron density, the CO₂ molecule becomes linear.

• Oxygen is more electronegative than carbon.
• One of the ”oxygen ends” is NOT more negatively charged than the other one ⇒ carbon dioxide is not a dipole.

The carbon dioxid molecule is linear.

How can you tell if a molecule is a dipole or not?

The VSEPR theory in short (explained in greater detail on this page):

• Look at the structure!
• Draw a Lewis formula.
• Count the areas with high electron density.
• Place the high-density areas as far apart from each other as possible – in 3D.

Ethylene glycol – Not necessarily a dipole, but still polar.

Space-filling model of an ethylene glycol molecule.

Ethylene glycol structure. The electronegative oxygen atoms cause local charge displacements within the molecule.

In the above configuration, the ethylene glycol molecule is not a dipole, because the positive and negative charges coincide with the center of the molecule.

It is, however, polar.

• The OH groups make the molecule polar.
• NH_x groups also make molecules polar.

Glucose and hexane

Glucose is polar because of its many OH groups.

Hexane does not contain any strongly electronegative atoms, and thus is non-polar.