Covalent Bonds: Sharing is Caring

Balancing Electrons

Unlike ionic bonds where electrons are transferred to and fro the electrons in covalent bonds are balanced between the two positively charged nuclei that pull on them. As a result, covalently bonded molecules form discrete entities unlike the crystalline lattices formed by ionic compounds.

Additionally, individual bonds can differ drastically depending on the two elements involved. This means that some bonds are longer and stronger than others. Furthermore, elements can share more than one electron at a time in an attempt to fill their valence shells. Resulting in elements that are bonded to the same atom multiple times as is seen by the formation of double or even triple bonds.

Length

The length of a covalent bond is determined by the distance between the two nuclei involved. Due to this, the atomic radii of the bonding elements determine to a large degree how long a bond is. For example, the bond between carbon and chlorine is much shorter than the bond between carbon and iodine. This occurs because chlorine is significantly smaller than iodine allowing the nuclei to get much closer to one another.

Additionally, a carbon-oxygen bond is slightly shorter than a carbon-carbon bond. Even though we aren’t used to thinking of bond length in terms of a periodic trend, we can. With bond length increasing as we go to the right and down matching the trend for atomic radii. With hydrogen-X element bonds being some of the shortest around.

Another factor that influences bond length is whether or not the bond is single, double, or triple. You can imagine each bond as an elastic band that pulls two nuclei closer together the more bands the closer the nuclei get. This means that single bonds are the longest, double bonds sit in the middle, and triple bonds are the shortest.

Strength

Coulomb’s law strikes again. Remember how distance was the biggest determining factor of the electrostatic force between charges. Well, this same fact that predicted atomic phenomena also predicts bonding phenomena. The closer the two nuclei are to the bonding electrons the stronger the force between them. This means that smaller elements can exert greater forces when in a bond and thus shorter bonds are stronger bonds.

\[
F=\frac{Kq_1q_2}{r^2}
\]

The same trend holds true for double and triple bonds too. If we follow through with our elastic band analogy this makes sense. A single bond only has one elastic band holding the atoms together as compared to a triple bond with three elastic bands. This means that single bonds are the weakest bonds and triple bonds are the strongest with double bonds sitting in the middle.

Energy

Due to the complexity of measuring bond strength using coulomb’s law or other force-based measurements we use bond energy instead. Bond energy is a measure of how much energy you need to rip a bond apart. So stronger bonds require more energy to break. Again with multiple bonds more elastic bands equals more energy to pull the nuclei apart. Thus single bonds have the smallest bond energies and triple bonds have the highest bond energies.

Bond energies are measured using a concept called enthalpy (∆H) or the measure of a system’s internal energy. We will see this concept come up again when we look at the thermodynamics of reactions which is concerned with quantifying the heat and energy of reactions.

Order

Lastly, we can describe covalent bonds by their bond order which is another way of attempting to quantify a bond’s overall strength. We are able to calculate this value, but it is time consuming, a bit of a pain, and extremely, extremely unlikely that you will be asked too.

[latexpage]
\[
Bond\;Order=\frac{Bonding\;Electrons-Antibonding\;Electrons}{2}
\]

Looking at this equation though we can see that more bonding electrons and less antibonding electrons equal higher bond orders. This means that the more electrons involved in a bond the higher the bond order. For instance, a single bond only involves two electrons while a triple bond involves six thus a triple bond would have a much higher bond order.

More broadly stronger bonds have higher bond orders, shorter bond lengths, and higher bond energies.

Question Strategies: Most-Least and Same-Different

On the MCAT we will be asked to determine which property belongs to . Typically we are asked to determine the strongest, weakest, longest, shortest, highest energy, lowest energy, largest bond order, or smallest bond order from a couple of options. With the most-least questions that ask about trends, we want to start thinking in extremes.

This means that we should eliminate any middling answers. For example, if a questions ask us to determine…

Furthermore