Covalent Bonds: Bond Polarity

By now we should realize that not all covalent bonds are created equal. Some are longer, weaker, and can involve different orbitals when bonding.

It’s Electron Tug of War

Another way that covalent bonds differ is in polarity or having a separation of charge within a bond or molecule. Since this whole lesson has been about bonds we are going to focus on how charges are separated within bonds and talk about molecules later.

Bonds are like a game of tug of war. An electron-rope holds two atoms together and the atoms pull against one another jostling for the bonding electrons. If one of the atoms pulls harder than the other it can bring the electrons in the bond closer to itself. This makes the “winning” atom slightly negative while making the “losing” atom slightly positive. Thus separating the charges making the bond polar.

Who Wins?

Since electronegativity is defined as the ability of an atom to pull on electrons in a bond, atoms with high electronegativities pull much harder than atoms with lower electronegativity values. That doesn’t mean that atoms with high electronegativities always win as it depends on the identity of their opponent.

For example, if two equally strong mice play tug of war the rope isn’t going anywhere and the same goes for two equally matched elephants. However, if a mouse and an elephant play tug of war we can say with confidence that the elephant is going to win (superhero mice aside). As a result, it is the electronegativity difference between two atoms that ultimately decides.

Let’s step out of our analogy now and put this into chemistry terms. We will do this by looking at four different covalent bonds. For all of these, we will focus on the absolute electronegativity difference.

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\[
EN\;Difference=|EN_1-EN_2|
\]

Hydrogen has an electronegativity of 2.1 so when two hydrogens are bonded to one another, as is the case in hydrogen gas, the absolute difference between them is zero. They are in a dead stalemate and the electrons are shared equally between the atoms. As a result, there is no separation of charge. When this occurs we say that the bond is non-polar or it doesn’t have separation of charge.

Again the same is true for the two oxygens below and again they would have an EN difference of 0 and as a result are nonpolar as well.

In a carbonyl bond, the picture changes. Here there is an absolute electronegativity difference of one in oxygen’s favor. This means that oxygen pulls the bonding electrons towards itself making oxygen partially negative while carbon is partially positive. Since there is a separation of charge this bond is polar. We will use a δ+ sign to denote the partially positive atom and a δ- sign to denote the partially negative atom.

The same is seen in an O-H bond. Here there is a larger absolute electronegativity difference of 1.4 in oxygen’s favor. Now fill in the blanks with what happens. Electrons are pulled to oxygen making oxygen partially___________ while hydrogen is partially __________. Since there is a ___________ of charge this bond is _________.

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Electrons are pulled to oxygen making oxygen partially positive while hydrogen is partially negative. Since there is a separation of charge this bond is polar.

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Revenge of Goldilocks

You might be wondering, “Why aren’t these just ionic bonds?” In an ionic bond, the electrons are ripped away from one atom whereas in a polar covalent bond the electrons are still shared albeit unequally. In terms of absolute electronegativity difference, a polar covalent bond is close to stealing the electrons but doesn’t pull quite hard enough to steal them.

This relationship is quantified using electronegativity values and you have probably seen these numbers in your general chemistry courses. In order for a polar covalent bond to exist its electronegativity difference has to be just right, not too big, not too small. You could memorize that chart below, but realistically it is unlikely to help you on the MCAT.

Bond Type	Electronegativity Difference
Nonpolar Covalent	<0.4
Polar Covalent	0.4 – 1.8
Ionic	>1.8

Instead, we want to focus on spot recognizing polar covalent bonds. There are two ways we can do this. First, by memorizing a list of common polar covalent bonds and second by using our general understanding of electronegativity values to predict what bonds are polar covalent.

We will focus on the first strategy later when we discuss particular types of molecules and the organic chemistry functional groups. For right now let’s explore using electronegativity values to determine bond type.

First up ionic bonds will form between halogens and alkali metals or alkaline earth metals. Their dramatically different electronegativities result in stealing rather than sharing.

Next, polar covalent bonds form between the upper right corner elements (N, O, F, S, Cl, and Br) excluding the noble gases and elements outside of this region. One notable exception are the bonds between the second-period elements with high electronegativity values. For example, a NO bond is polar as is a OF bond.

Lastly, nonpolar bonds form between identical elements and between elements excluding those is in the upper right corner. For example, a CH bond is nonpolar as is an oxygen oxygen bond.