observed bond lengths

[Vicky Tong 3E]

Are the observed bond lengths given to us or is this something that we need to know off the top of our heads? I'm not entirely sure that I understand this concept. What exactly is expected/observed bond lengths and how are they used in problems?

[Lucas Koch 1I]

Observed bond lengths are the length from center to center of two bonded atoms. We don't have to know specific numbers but we need to know related concepts such as double bonds will always have shorter bond lengths than single and triple bonds will be shorter than both. It is also important to know that when a molecule or polyatomic ion has resonance the bond lengths will be between single and double if both types are possible. Again, we won't need to know specific numbers unless they are given, but we may need to understand relative lengths.

[Jennifer Huynh 3I]

The observed bond lengths are obtained from experimentation, and they vary from our expected bond lengths that are known from measurements of bonds we know for certain exist between two atoms. The example given in lecture was that we know the double bond between C and C is 1.34 Angstroms and the single bond between C and C is 1.54 Angstroms, but our observed bond lengths for all of the C-C bonds in C6H6 are between the two expected lengths (1.39 Angstroms). The difference between the expected and observed bond lengths shows that Lewis structures are models that can help us understand molecular structures, but they are not perfect or always exact. So, we know that resonance structures are a combination of all the possible Lewis structures for a specific molecule.

[Isabelle Kim 3E]

I thought the whole concept behind bond lengths is generally correlated to the atomic radius trend. So, it would follow the atomic radius trend by decreasing across the period and increasing down the group. For atoms with multiple bonds, the overall bond lengths would be expected to be shorter. Also, the bond length is measured to be the AVERAGE due to the constant vibration of the atoms.

[emmakvarnell]

Observed bond lengths in a problem should be given to us. You should basically just be able to compare different given bond lengths in order to identify which is a single or double bond. The difference between expected and observed is just that in a molecule with resonance, the expected bond length should be specific to either a double or single bond, depending on which atoms have the double bond in the model. The observed bond length just refers to the experimentally found bond length which shows how resonance gives each atom a partial double bond.

[Rio Gagnon 1G]

We don't need to know specific numbers from memory; however, you should know that the more bonds, the shorter they are (ex: triple bond shorter than a double bond, which is shorter than a single bond). Also, know that for resonance structures, if there is a mix between single and double bonds (etc), the bond length is actually the average of the different bond types.

[Sonia Virk 2A]

No, we do not have to know the measurements for the bond lengths. However, we need to conceptually know that single bonds are longer than double and triple bonds and so forth.