I just learned that there are three kinds of overlapping (positive, negative and zero). I don't thoroughly understand the concept of different signs in orbitals and their overlappings. What does difference would it make if the signs are different (negative overlapping happens) when they don't really play a significant role? Could someone explain how waves and orbital shapes along with overlapping in brief? 

5 hours ago, mundane said:

I just learned that there are three kinds of overlapping (positive, negative and zero). I don't thoroughly understand the concept of different signs in orbitals and their overlappings. What does difference would it make if the signs are different (negative overlapping happens) when they don't really play a significant role? Could someone explain how waves and orbital shapes along with overlapping in brief? 

OK, I'll have a go at this. The signs are phases of the orbital wave function, just as you have a +ve and -ve phase in an alternating current, or in a water wave as crests (+) and troughs (-)  pass you.

When combining atomic orbitals to make a bond there is either the option of the two having the same phase (one is + when the other is +), or of them having opposite phase (one is + when the other is -). If they have the same phase, the resulting combined orbital has a build up of electron density between the atoms, has lower energy than either of the atomic orbitals, and is therefore a bonding orbital. If the phases are opposite, you have a node between the two atoms (where the sign of the phase changes), indicating that electron density is reduced, rather than increased, in the region between the atoms, and this corresponds to an antibonding orbital, which is of higher energy than either atomic orbital. Both bonding and antibonding orbitals form when 2 atoms approach one another. A chemical bond will form if there are enough electrons to populate the bonding orbital but not the antibonding one.  (An example of where both are populated is when 2 inert gas atoms approach one another. Both orbitals are populated and the repulsion due to the antibonding one cancels the attraction from the bonding one, so no bond forms.)