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Topic: Valence bond theory and resonance  (Read 228 times)

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Offline Ranjan Bassi

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Valence bond theory and resonance
« on: January 13, 2020, 02:51:27 AM »
So, I am currently in highschool and read about valence bond theory and also about resonance. In my textbook there is mentioned the a molecule, when in resonance, is not in two different canonical form but rather in a hybrid structure at all times. So how do we integrate resonance into the valence bond theory and hybridization. Do the hybridized orbitals exist in different shapes for resonating structure or what? Then on the bases of this, how to we explain the geometry of resonating structures?( I am a little weak in chemistry so feel free to correct me if I misinterpreted something).

Offline Enthalpy

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Re: Valence bond theory and resonance
« Reply #1 on: January 13, 2020, 11:06:42 AM »
Welcome, Ranjan Bassi!

Bonds between two atoms, hybridisation and resonances are an old theory. It's knowingly wrong, but it still commonly used because it's fertile and its simplicity is manageable, so you have to learn and know it. Molecular orbitals is the presently correct theory, but reasoning with it is difficult, computing even more.

So don't worry too much if the old theory is a bit rickety. That's normal and known. Resonances and other patches were added in an attempt to save that theory.

Yes, the electronic configuration of most molecules under usual conditions is static. It does not evolve over time. Quantum mechanics calls that "stationary", which is nearly the same as "immobile". That's why electrons don't radiate light.

In a first approximation, when the possible states of a molecule have energies far apart, the molecule is in the state of lowest energy, especially if the ambient temperature doesn't suffice to reach the next energy state. This holds often for electronic states, with exceptions. Sometimes the vibrations of the molecule (when atoms move) are accessible to the ambient temperature and are excited. The rotations are usually excited as the possible states are closer to an other than the ambient temperature.

The static nature of a resonance explains you why it stabilizes an ion for instance. A charge spread permanently over more atoms hence volume needs less electrostatic energy. If the charge hopped between atoms, it would still have the same electrostatic energy as if it were permanently on a single atom.

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