[delphinas]

Hello everyone,
I'm currently studying organic chemistry. I try to use various resources (books, internet, etc) to find something I don't get. However I've spent some time reading through molecular theory and orbitals. I got stuck there!

I do not understand the bonding and anti-bonding orbital concept.
Picture to clarify:

So are these two bonding and anti-bonding MO both occurs at the same time in a molecule? OR there is only a possibility that when two atoms react they could form either bonding MO or anti-bonding MO. ?

Also why these two orbitals are at different energy levels?

Thank you for your answers.

[IsotopeBill]

Yes, both the bonding and antibonding orbitals form at the same time.  However, what is significant is not that they form, but whether or not they get occupied.  As you've drawn, both the electrons fill the lower-energy, bonding MO, leaving the higher-energy, antibonding MO unoccupied.  If one of your 1s orbitals had two electrons present, then the bonding MO would have two electrons and the antibonding a single electron, making the chemical bond weaker than one with an unpopulated antibonding orbital (since there would be an electron in the antibonding MO).  If two atoms are to bond, then there must be able to form a bonding MO at lower energy than the atomic orbitals of the two starting atoms (otherwise there's no incentive for the atoms to bond together).
This explanation is a little overly-simplistic, but it's a start

[delphinas]

Thank you for your answer!
Maybe I over complicating this, but I can't get my head around how two different MO (bonding and anti-b) can form from  let's say 1s and 1s in the same one molecule. I mean logically if you combine 2 things, you suppose to get 1 conjugated thing. What is a proper scientific explanation for these two MO to form?

*I know the background that the orbitals are just regions where electrons can be found, and that electrons acts as a wave or a particle.

Thanx again!

[IsotopeBill]

Molecular orbitals aren't "things" in the tangible sense; your definition of them as "regions" is probably more accurate.  It's difficult for us to imagine the quantum realm and to give concrete examples using everyday objects, because the rules seem to be all different.  However, you're starting out with two 1s atomic orbitals.  Their wavefunctions mix, and you end up with two molecular orbitals.  The atomic orbitals are identical, and there are two of them.  The resulting molecular orbitals are not identical (in fact, you could say they are "opposite"), but there are still two of them.  So, in a way, you start out with two "things", and you end up with two "things".

You probably know the story of Schroedinger's cat:  Schroedinger puts his cat in a box, along with a nefarious machine that may or may not kill the cat:  whether it does or not is purely random.  He closes the lid to the box, waits the allotted time, then prepares to open the box.  (Please don't try this experiment at home:  you'll have a lot of explaining to do.)  Immediately before the box is opened and the state of the cat is observed, the cat actually exists in two states simultaneously.  That is, there is a "dead cat" state and a "live cat" state.  It's a bit more than there being an equal probability of the cat being dead or alive, but not so much that there are two actual cats (a dead one and a live one).  The state of the cat is both dead and alive, and the actual result is dependent upon the cat being observed by Schroedinger. Once he opens that box, he will have either a fluffy ball of joy or a fetid corpse, but before the observation, he has neither.  Or both, depending on how you look at it.  This cat allegory is useful, because it illustrates the difficulty in using our perception of the world when trying to think about the quantum realm.
  In a way, your bonding and antibonding orbitals are like the state of the cat immediately before the box is opened, and your electrons - or rather your pair of electrons - are like the observer, except in this case you always "observe" the lower-energy state, because the electrons populate the lower-energy level.  (Actually I think there's a statistical factor involved in that there is some degree of population of the higher-energy antibonding orbital, but I'm not really sure.)  The atoms approach each other, and their wavefunctions mix to give two states, a lower-energy bonding state,and a higher-energy antibonding state, with the lower-energy state being filled first.  So in the case you've drawn, you'll always have a happy cat, and a diatomic molecule.
   Milo Wolff has written a lot about this stuff, and I recommend visiting his website and the sites he has links to, if you're at all interested in this

[orgopete]

So are these two bonding and anti-bonding MO both occurs at the same time in a molecule? OR there is only a possibility that when two atoms react they could form either bonding MO or anti-bonding MO. ?

Also why these two orbitals are at different energy levels?

I prefer to take a very simplistic approach to this question. Let us assume the diagram denotes the spins of the electrons. If the spins are opposite, then the electrons can form a bonding, lower energy, configuration. If the spins are parallel, the electrons behave normally, repel one another, and form an anti-bonding configuration.

[souro10]

I think the poster wants to know why two atomic orbitals combine to give two molecular orbitals , and why they differ in energy.

Suppose a 1s orbital of Hydrogen atom is combining with another 1s orbital of the other Hydrogen atom. Orbitals, as you know, can be are wave functions of the atoms. Your elementary physics classes must have already taught you about phase of waves. Simply put, orbitals also have phase, just like the waves. And just like waves, they can combine in-phase , or out-of-phase. When two waves in the same phase superpose, what happens? They combine constructively to form a resulting wave that has higher amplitude. And what happens when two waves combine out-of-phase? They combine destructively, and in case of atomic orbitals , they lead to formation of a node



" When the two orbitals combine out-of-phase, the resulting molecular orbital has a nodal plane
between the two nuclei. This means that if we were to put electrons into this orbital there would be
no electron density in between the two nuclei. By contrast, if the molecular orbital from in-phase
combination contained electrons, they would be found in between the two nuclei. Two exposed
nuclei repel each other as both are positively charged. Any electron density between them helps to
bond them together. So the in-phase combination is a bonding molecular orbital. As for the electrons
themselves, they can now be shared between two nuclei and this lowers their energy relative to
the 1s atomic orbital. Electrons in the orbital from the out-of-phase combination do not help bond
the two nuclei together; in fact, they hinder the bonding. When this orbital is occupied, the electrons
are mainly to be found anywhere but between the two nuclei. This means the two nuclei are more
exposed to each other and so repel each other. This orbital is known as an antibonding molecular
orbital and is higher in energy than the 1s orbitals. "

The quoted extract and the image is from the Organic Chemistry textbook by Jonathan Clayden and Co-authors. I think that answers your question.