Polar bonds and molecules

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Covalent bonds involve electron sharing between atoms. However, covalent bonds differ in terms of how the bonded atoms share the electrons.

The character of the bonds in a molecule depends on the kind and number of atoms joined together. These features determine the molecule’s properties.

The bonding pairs of electrons in covalent bonds are pulled, as in a tug-of-war, between the nuclei of the atoms sharing the electrons.

When the atoms in the bond pull equally (as occurs when identical atoms are bonded), the bonding electrons are shared equally, and the bond is a nonpolar covalent bond.

Molecules of hydrogen (H2), oxygen (O2), and nitrogen (N2) have nonpolar covalent bonds.

Diatomic halogen molecules, such as Cl2, are also nonpolar.

(Image above from Florida Dept of Education)

A polar covalent bond is a bond between atoms in which the e- are shared unequally.

The more electronegative atom attracts e- more strongly and so gains a slightly negative charge.

The less electronegative atom has a slightly positive charge.

Consider the hydrogen chloride molecule (HCl)

Cl has a higher electronegativity (3.0), so it pulls e- more towards it.

H has a lower electronegativity (2.1), so it holds e- less.

As a result, more of the e- cloud is towards the Cl atom.

Therefore the molecule is polar – more charged on one side than the other.

The lower-case Greek letter delta (δ) means “partial electric charge.”

δ+ means that there’s a lack of negative charge locally, so the atom is a bit positive.

δ- means that there’s an excess of negative charge locally, so the atom is a bit negative.

The polar nature of the bond may also be represented by an arrow pointing to the more electronegative atom, as shown here.

O⎯H bonds in the water molecule are polar.

The highly electronegative O partially pulls the e- cloud away from H.

The O acquires a slightly neg charge. The H is left with a slightly pos charge.

How electronegativity differences are related to bond types

The electronegativity difference between two atoms tells us what kind of bond is likely to form.

(Note: There’s no sharp boundary between ionic and covalent bonds.)

As the electronegativity difference between atoms increases, the polarity of the bond increases.

If the difference is greater than 2.0, it is very likely that 1 or more e- will be pulled away completely by one of the atoms. In that case, an ionic bond will form

Dipoles and polar molecules

In a polar molecule, one end of the molecule is slightly neg and the other end is slightly pos.

Such a molecule is called dipolar (having two “poles,” although this is electrical polarity, not magnetic.)

HCl is dipolar

So is H2O, water. The oxygen atom is more electronegative so it attracts more of the e- cloud; the H atoms have less of the e- cloud, so they are relatively positive.

What about CO2, carbon dioxide?

How polar molecules react to electric fields

Imagine two metal plates, connected to a battery as shown below. The battery pulls some charges (valence electrons) away from the plate on the left (leaving it with a net + charge) and pushes them to the plate on the right (leaving it with a net – charge.)

In electronics this is called a parallel plate capacitor, and it has direct applications everywhere in physics, biology, and chemistry.

If viewed as a machine, we could say that it stores electrical charge and energy; it generates an electric field between the plates (the direction of the field is shown here as red lines)

This electric field isn’t an “idea” or concept – fields – like electric fields or gravitational fields – are just as real as particles. We can see how they affect objects.

This image from Capacitors and Capacitance, Physics LibreTexts

What would happen if we put polar molecules (dipoles) in between these plates?

On the left the plates are not charged; there is no electric field.

On the right the plates are charged; the left plates becomes – and the right plate becomes +. This creates an electric field between the plates.

In response, the polar molecules respond to the field by orienting themselves: their – ends are attracted to the + plate (and vice-versa.)