Polar and Non-Polar Covalent Bonds

What are Polar and Non-Polar Covalent Bonds?

A covalent bond is nothing but a shared pair of electrons. A covalent bond is formed when atomic orbitals from two atoms, usually non-metals, overlap, and the electrons within them form a pair that is shared by both atoms. The bond is held together by strong electrostatic attraction between the negative electrons and the atoms' positive nuclei.

If the two atoms involved in the covalent bond are the same, they share the electron pair evenly between them. This forms a non-polar bond.

One example is hydrogen gas, H₂. The two hydrogen atoms are identical, so the bond between them is non-polar.

Fig. 1. A non-polar H-H bond.

But if the two atoms involved in the covalent bond are different, the electron pair might not be shared evenly between them. One atom could attract the shared pair of electrons more strongly than the other atom, pulling the electrons over towards itself. The electron pair is shared unequally between the two atoms. We call this a polar bond.

Now we know that a polar bond is formed when an electron pair is shared unequally between two atoms. But what causes this uneven distribution?

What Causes Polar Bonds?

We've learned that polar covalent bonds are formed when one atom in a covalent bond attracts the shared pair of electrons towards itself more strongly than the other. This is all to do with the atom's electronegativity.

We measure electronegativity on the Pauling scale. It runs from 0.79 to 3.98, with fluorine being the most electronegative element, and francium the least electronegative. (The Pauling scale is a relative scale, so don't worry about how we get these numbers for now).

Fig. 2. The Pauling scale.

When it comes to covalent bonds, the more electronegative atom attracts the shared pair of electrons more strongly than the less electronegative atom. The more electronegative atom becomes partially negatively charged, and the less electronegative atom becomes partially positively charged. For example, you can see in the table above that oxygen is a lot more electronegative than hydrogen. This is why the oxygen atom in an O-H bond becomes partially negatively charged, and the hydrogen atom becomes partially positively charged.

In general, we can say the following:

• When two atoms with the same electronegativity share a pair of valence electrons, they form a non-polar bond.
• When two atoms with different electronegativities share a pair of valence electrons, they form a polar bond.

Characteristics of Polar and Non-Polar Covalent Bonds

Now that we know what polar and non-polar covalent bonds are, let's look at their characteristics. In the section above, you learned that polar covalent bonds are formed between two elements with differing electronegativities. This gives polar covalent bonds the following characteristics:

• The atoms have partial charges.
• The molecule has a dipole moment.

One example of a polar bond is the O-H bond, such as in water, or H₂O. Oxygen attracts the shared pair of electrons much more strongly than hydrogen, resulting in a polar bond. Let's use this example to explore the characteristics of polar covalent bonds a little further.

Partial Charges

Look at our example, the O-H bond. Oxygen is more electronegative than hydrogen and so attracts the shared pair of electrons towards itself more strongly. Because the negative pair of electrons is found much closer to oxygen than hydrogen, the oxygen becomes partially negatively charged. The hydrogen, which is now electron-deficient, becomes partially positively charged. We represent this using the delta symbol, δ.

Fig. 3. The polar O-H bond.

Dipole Moments

You can see in the example above that the uneven distribution of electrons in a polar bond causes an uneven distribution of charge. One atom involved in the bond becomes partially negatively charged, while the other is partially positively charged. This creates a dipole moment. Asymmetrical molecules with dipole moments form dipole molecules. (You can explore this in more detail in Dipoles, and Dipole Moment.)

The Difference Between Polar and Non-Polar Covalent Bonds

The basic difference between a polar and non-polar covalent bond is that a polar covalent bond has an unequal distribution of charges, whilst in a non-polar bond all atoms have the same charge distribution. This is because in polar bonds some of the atoms have higher electronegativity than others, whilst in the non-polar bonds all atoms have the same electronegativity value.

However, in real-life examples, when it comes to bonding, it is hard to draw a line between polar, non-polar, and indeed even ionic bonding. To understand why, let's look more closely at one particular bond: the C-H bond.

Visit Ionic Bonding for more on this subject.

Bonding falls on a spectrum. At one end, you have completely non-polar covalent bonds, formed between two identical atoms with the same electronegativity. At the other end, you have ionic bonds, formed between two atoms with an extremely large difference in electronegativity. Somewhere in the middle, you find polar covalent bonds, formed between two atoms with an intermediate difference in electronegativity. But where do we draw the limits?

• If two atoms have an electronegativity difference of 0.4 or less, they form a non-polar covalent bond.
• If two atoms have an electronegativity difference between 0.4 and 1.8, they form a polar covalent bond.
• If two atoms have an electronegativity difference of more than 1.8, they form an ionic bond.

We can say that the bond has an ionic character proportional to the difference in electronegativity between the two atoms. As you might be able to guess, atoms with a larger difference in electronegativity show more ionic character; atoms with a smaller difference in electronegativity show less ionic character.

Fig. 4. Non-polar, polar, and ionic bonds are shown with the electronegativities of the atoms.

Predicting Bonding from Elemental Properties

Although bonding falls on a spectrum, it is often easier to classify a bond as non-polar covalent, polar covalent, and ionic. Generally, a bond between two non-metals is a covalent bond, and a bond between a metal and a non-metal is an ionic bond. But this isn't always the case. For example, take SnCl₄. Tin, Sn, is a metal, and chlorine, Cl, is a non-metal, so we'd expect them to bond ionically. However, they actually bond covalently. We can use their properties to predict this.

• Ionic compounds have high melting and boiling points, are brittle, and can conduct electricity when molten or aqueous.
• Covalent small molecules have low melting and boiling points and don't conduct electricity.

Let's look at our example above: SnCl₄ melts at -33°C. This gives us a pretty good indication that it bonds covalently, not ionically.

You might wonder: Why don't we just look at the difference in electronegativity when determining the nature of a bond? While it is a useful guide most of the time, this system doesn't always work.

To summarise, ionic bonding is generally found between metals and non-metals, and covalent bonding is generally found between two non-metals. Electronegativity differences also give us an indication of the bonding present in a molecule or compound. However, some compounds break these trends; looking at properties is a more reliable way of determining the bond.

List of Polar and Non-Polar Covalent Bonds (Examples)

Let's end with some examples of polar and non-polar covalent bonds. Here's a handy table that should help you.

That's all! You should now be able to state the difference between polar and non-polar covalent bonding, explain how and why polar bonds are formed, and predict whether a bond is polar or non-polar based on the properties of the molecule.