Question

Select the stronger bond in each pair: (a) \(\mathrm{Cl}-\mathrm{Cl}\) or \(\mathrm{Br}-\mathrm{Br}\) (b) \(\mathrm{Br}-\mathrm{Br}\) or \(\mathrm{I}-\mathrm{I}\) (c) \(\mathrm{F}-\mathrm{F}\) or \(\mathrm{Cl}-\mathrm{Cl}\). Why doesn't the \(\mathrm{F}-\mathrm{F}\) bond strength follow the group trend?

Short answer

In each pair, the stronger bond is Cl-Cl, Br-Br, and Cl-Cl respectively. The F-F bond strength does not follow the trend due to electron repulsion.

Step-by-step solution

Understand bond strength

Bond strength refers to the energy required to break a bond between two atoms. Generally, as you move down a group in the periodic table, bond strength decreases because atomic size increases, leading to weaker bonds.

Compare Cl-Cl and Br-Br

Chlorine (Cl) is above Bromine (Br) in the periodic table. Since Cl-Cl has a smaller atomic size than Br-Br, the bond between Cl atoms is shorter and stronger than the bond between Br atoms.

Compare Br-Br and I-I

Bromine (Br) is above Iodine (I) in the periodic table. As Br is smaller than I, the bond in Br-Br is shorter and stronger than that in I-I.

Compare F-F and Cl-Cl

Fluorine (F) is above Chlorine (Cl) in the periodic table. Following the general trend, one would expect F-F to be stronger than Cl-Cl. However, F atoms are very small and have their electron pairs very close, causing repulsion and making the F-F bond exceptionally weak.

Explain the exception for F-F bond

Despite being higher in the group, the F-F bond is weaker than the Cl-Cl bond due to the high electron-electron repulsion in the small F molecule. This repulsion is not predominant in the larger Cl molecule, making Cl-Cl stronger.

Key concepts

Periodic Table Trends

The periodic table is a crucial tool in chemistry. It helps us understand trends and predict properties of elements.
One important trend is that as you move down a group (column) in the periodic table, the size of atoms increases. This is due to the addition of extra electron shells.
Another major trend is that bond strength typically decreases as you move down a group. This is because the atoms are getting larger, and the bonds between them become longer and weaker.
In our specific exercise, this helps us understand why chlorine (Cl), which is above bromine (Br), forms a stronger bond than bromine itself does.
Chlorine is smaller, which leads to a shorter and stronger bond.

Atomic Size

Atomic size, or atomic radius, is a measure of the size of an atom.
As mentioned, atomic size increases as we move down a group in the periodic table. This means that atoms at the top of the group are smaller than those at the bottom.
For example: Fluorine (F) and chlorine (Cl) are both in Group 17, but fluorine is smaller than chlorine. This information is essential to understand why some bonds are stronger than others.
Smaller atoms like fluorine and chlorine form shorter and typically stronger bonds because their nuclei are closer together, allowing for a stronger attraction of the bonding electrons.

Electron Repulsion

Electron repulsion occurs when negatively charged electrons are too close to each other. This can weaken the bond between two atoms.
For small atoms like fluorine, the electrons are very close together, leading to significant repulsion.
This repulsion can make the bond weaker, even if the atoms themselves are small. This is why the F-F bond is weaker than what we would expect from its position in the periodic table trend.
The closer the electron pairs are, the greater the repulsion. That's why the F-F bond is an exception to the trend we notice in other group 17 elements.

Bond Energy

Bond energy is a measure of the strength of a chemical bond.
It is the amount of energy required to break a bond between two atoms. Stronger bonds require more energy to break.
In general, shorter bonds (found in smaller atoms like chlorine) are stronger and have higher bond energies.
However, we see exceptions with high electron repulsion, such as in the F-F bond, where despite the small size, the bond is weaker due to the repulsion between electrons.
Understanding bond energy helps in predicting the chemical behavior and stability of molecules.