Molecular orbital theory and the conservation of molecular orbitals 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?
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.