Poll 2 - Answers

In this poll, you were asked:

Which of the following factors is the cause for Br-Br single bond to be weaker than the Cl-Cl single bond.

The following are the choices:

1. The Br atom is larger than the Cl atom.
2. The Br-Br bond length is longer than the Cl-Cl bond length.
3. The electrostatic forces of attraction between the shared electrons and nuclei is weaker in the Br bond than the Cl bond.
4. None of the above.

The answer.

Choice 3.

Reasoning:

The definition of covalent bond is the electrostatic forces of attraction between shared electrons and nuclei. Hence, if the bond is weaker it must be due to the electrostatic forces of attraction between the shared electrons and nuclei weaker.

Of course, the larger atom plays a role. Larger atoms causes the shared electrons to be placed further away from the nucleus and hence the attractive force would be weak. But once again, it is the weak electrostatic force that causes the chemical bond to be weak, if there is no electrostatic force, there will not be a bond to discuss in the first place.

Needless to say, it is because of the weaken electrostatic attraction that causes the bond length to be longer. Thus, bond length should be seen as a resultant of bond strength and not a cause to bond strength.

Rationale:

This post serves as a reminder of the definition of a covalent bond. It's definition is extremely useful in conceptualising why formation of stronger bonds causes the reaction to be more exothermic (This is because the product will be energetically more stable than the reactant).

In addition, this also explains why if reactants have strong bonds, its activation energy is usually very large.