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When the two 1 s wave functions combine out-of-phase, the regions of high electron probability do not merge. This second orbital is therefore called an antibonding orbital. The other, corresponding to out-of-phase combination of the two orbitals, gives rise to a molecular orbital that has its greatest electron probability in what is clearly the antibonding region of space. The in-phase, reinforcing interaction yields the bonding orbital that we just considered. A roughly similar thing occurs when the “matter waves” corresponding to the two separate hydrogen 1 s orbitals interact both in-phase and out-of-phase combinations are possible, and both occur.

You are probably aware that wave phenomena such as sound waves, light waves, or even ocean waves can combine or interact with one another in two ways: they can either reinforce each other, resulting in a stronger wave, or they can interfere with and partially destroy each other. How can we find the missing orbital? To answer this question, we must go back to the wave-like character of orbitals that we developed in our earlier treatment of the hydrogen atom. This is just another way of saying that there must always be the same number of possible allowed sets of electron quantum numbers.

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For one thing, this would raise the question of at just what internuclear distance do we suddenly change from having two orbitals, to having only one? It turns out that when orbitals interact, they are free to change their forms, but there must always be the same number. Now according to the rules of quantum mechanics, orbitals cannot simply appear and disappear at our convenience. There is one minor difficulty: we started with two orbitals (the 1 s atomic orbitals), and ended up with only one orbital.

The corresponding orbitals will then be the molecular orbitals of our new molecule.īonding and Antibonding Molecular Orbitals Finally, we will reach some point where the internuclear distance corresponds to that of the molecule we are studying. We will then try to predict the manner in which these atomic orbitals interact as we gradually move the two atoms closer together. These are just the orbitals of the separate atoms, by themselves, which we already understand. The easiest way of visualizing a molecular orbital is to start by picturing two isolated atoms and the electron orbitals that each would have separately. Conversely, if the electron is off to one side, in an anti-binding region, it actually adds to the repulsion between the two nuclei and helps push them away. For this to happen, the electron must be in a region of space which we call the binding region. But all of these valence-bond models, as they are generally called, are very limited in their applicability and predictive power, because they fail to recognize that distribution of the pooled valence electrons is governed by the totality of positive centers.Ĭhemical bonding occurs when the net attractive forces between an electron and two nuclei exceeds the electrostatic repulsion between the two nuclei. The more sophisticated hybridization model recognized that these orbitals will be modified by their interaction with other atoms. This is a big departure from the simple Lewis and VSEPR models that were based on the one-center orbitals of individual atoms.

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In its full development, molecular orbital theory involves a lot of complicated mathematics, but the fundamental ideas behind it are quite easily understood, and this is all we will try to accomplish in this lesson. The molecular orbital model is by far the most productive of the various models of chemical bonding, and serves as the basis for most quantiative calculations, including those that lead to many of the computer-generated images that you have seen elsewhere in these units.

Construct a 'molecular orbital diagram' of the kind shown in this lesson for a simple diatomic molecule, and indicate whether the molecule or its positive and negative ions should be stable.

Define bond order, and state its significance.

Describe the essential difference between a sigma and a pi molecular orbital.

Explain how bonding and antibonding orbitals arise from atomic orbitals, and how they differ physically.

In what fundamental way does the molecular orbital model differ from the other models of chemical bonding that have been described in these lessons?.

Make sure you thoroughly understand the following essential ideas

Lithium atom model