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I am not clear with the molecular orbital theory. I have read:

If the atomic orbitals are combined with the same phase they interfere constructively and a bonding orbital is formed.
If the atomic orbitals are combined with different phases, they interfere destructively and an anti-bonding molecular orbital is formed.

So as per my understanding when two s-orbitals are combined either a bonding orbital can be formed or an anti-bonding orbital can be formed depending upon the type of interference. This contradicts the following statement:

When two atomic orbitals combine they form two new molecular orbitals.

This means that when two s-orbitals combine they produce both bonding and anti-bonding orbitals. Where have I gone wrong?

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    $\begingroup$ The problem is that you interpret the notion of "orbital interference" literally. That is not your fault: many texts are still written in a way so that they give a false impression that orbitals indeed interfere with each other. But they do not. Strictly speaking, orbitals do not even exist, but that is a bit different story. Orbitals combine in a certain way which is just somewhat similar to wave interference, but is not wave interference. $\endgroup$ – Wildcat Jun 23 '16 at 14:40
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    $\begingroup$ I would agree with 'wildcat' that thinking of interference is not particularly helpful in this case. Thinking of combining orbitals of the same 'phase' is more useful as a Rule of Thumb. You are right in thinking that two orbitals produce a bonding and anti-bonding orbital. The sum of these orbital energies is the same as the sum of the original orbitals. Putting electrons in the bonding orbital lowers to total energy. $\endgroup$ – porphyrin Jun 25 '16 at 8:06
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The two statements are not contradictory. There are indeed two ways to combine the two atomic orbitals to form new molecular orbitals. It is not either one way or the other--in fact both occur - one way creates a bonding molecular orbital and the other creates an antibonding molecular orbital.

So for example, if you have two ground state hydrogen atoms, each with an electron in the 1s orbital, you end up with a hydrogen molecule, with 2 electrons in the lower energy (sigma) bonding molecular orbital and 0 electrons in the (sigma*) antibonding molecular orbital. The same thing is true for formation of pi bonds from p orbitals.

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  • $\begingroup$ But how can two waves interfere both constructively and destructively at the same time? $\endgroup$ – oshhh Jun 23 '16 at 11:36
  • $\begingroup$ Here is a way to think about it: an interference pattern formed by combining two waves has regions of constructive interference and other regions of destructive interference. $\endgroup$ – iad22agp Jun 23 '16 at 12:00

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