When does resonance take place?

In summary, resonance structures can occur when you have a conjugated system of atoms that can delocalize electrons over more than two atoms.
  • #1
kay
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As in what are the conditions for resonance (i.e. in oganic chemistry) to occur? Sometimes it occurs, sometimes it doesn't; for reasons unknown. A little help please? :/:olduhh:
 
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  • #2
kay said:
As in what are the conditions for resonance (i.e. in oganic chemistry) to occur? Sometimes it occurs, sometimes it doesn't; for reasons unknown. A little help please? :/:olduhh:

Generally, if you can draw different Lewis structures with different arrangements of double bonds, then the different structures are resonance structures. Some resonance structures may be more or less important. All resonance structures are not create equal -- some represent higher energy structures.
 
  • #3
Quantum Defect said:
Generally, if you can draw different Lewis structures with different arrangements of double bonds, then the different structures are resonance structures. Some resonance structures may be more or less important. All resonance structures are not create equal -- some represent higher energy structures.
What i meant to say was that when does resonance occur? what are the conditions necessary for resonance to take place? I'm not asking how to draw resonating structures. My mind was searching for the fact on the basis of which i would be able to decide whether i COULD make resonating structures or not
 
  • #4
kay said:
What i meant to say was that when does resonance occur? what are the conditions necessary for resonance to take place? I'm not asking how to draw resonating structures. My mind was searching for the fact on the basis of which i would be able to decide whether i COULD make resonating structures or not

I don't know of any ready way (without drawing resonance structures) to tell, a priori, that resonance structures are there. Generally, you need some multiple bonds, possibly some lone pairs. But the presence of these does not guarantee that resonance structures are important to describe the molecule.

You can see experimental evidence of resonance if you know things like bond lengths for your structure that you can compare to typical bond lengths. I.e. if the experimental bond length is intermediate between a typical single bond and a typical double bond, you are likely to have resonance structures where the bond in question is either single, or double, depending upon the resonance stucture.

The presence of resonance structures may also be revealed through other experimental measurements (dipole moments, thermochemical data, etc.)
 
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That's kind of a difficult question. In general, you need much more resonance structures than you are usually considering in introductory classes. E.g. to describe a simple molecule like ##\mathrm{H_2}## starting from its atomic orbitals you need to include H-H, H##^+##H##^-## and H##^-##H##^+##.
 
  • #7

1. When does resonance occur?

Resonance occurs when an object is subjected to a periodic force at the same frequency as its natural frequency.

2. What is the difference between resonance and vibration?

Vibration refers to the back and forth motion of an object, whereas resonance is the phenomenon of an object vibrating at its natural frequency in response to an external force at the same frequency.

3. How does resonance affect structures?

Resonance can cause destructive vibrations in structures, leading to potential damage or collapse. It is important to consider resonance when designing and building structures to prevent these effects.

4. Can resonance be beneficial?

Yes, resonance can also be used to our advantage in various technologies such as musical instruments, radio receivers, and MRI machines. In these cases, resonance is carefully controlled and harnessed for its intended purpose.

5. How can resonance be avoided?

Resonance can be avoided by ensuring that the frequency of any external force is different from the natural frequency of the object. This can be achieved by using dampers or changing the material or design of the object.

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