[Kalibasa]

They told us in intro chem that bond order is, in simplified terms, the number of bonds between electrons. In molecular orbital language, though, they say that bond is (the number of bonding electrons - the number of antibonding electrons)/ 2.

I don't understand why antibonding electrons would get subtracted. I'd thought antibonding was just a different kind of bond, but if these elecrtons are getting subtracted, does this mean that antibonding isn't a real bond? Is it actually forcing the atoms apart?

I'm really confused...

Thanks!

[stewie griffin]

They told us in intro chem that bond order is, in simplified terms, the number of bonds between electrons.

I think you mean atoms, not electrons.
Anyway, antibonding means the opposite of bonding. An antibonding orbital is just another type of orbital but one that is destructive to bonding (or as you way "forcing the atoms apart"). There is no such thing as a bond made up of solely antibonding orbitals. A bond is made up instead of bonding orbitals (these are the ones we refer to when we say two atoms are bonded together) and antibonding orbitals.
Since antibonding electrons are destructive to bond formation, they are subtracted when determining the bond order.

[Ligander]

I don't understand why antibonding electrons would get subtracted. I'm really confused...

Thanks!

 To soothe yourself treat it as a definition which has to be consistent with valence bond theory definition.

 If you do not subtract antibonding electrons "MO bond order" might be incosistent with "VBT bond order".

[Schrödinger]

I guess we could look at it this way:

The bonding electrons are the ones that contribute to the net attraction b/w atoms.
The antibonding electrons are those that contribute to the repulsion b/w atoms.

(This is because, when there is destructive interference of orbitals, the net electron density decreases between the nuclei. This make the nuclei more 'exposed' to each other than they are when 'shielded' by the electrons b/w them. This causes repulsion.)

Please correct me if I am wrong

[Ligander]

I guess we could look at it this way:

The bonding electrons are the ones that contribute to the net attraction b/w atoms.
The antibonding electrons are those that contribute to the repulsion b/w atoms.

(This is because, when there is destructive interference of orbitals, the net electron density decreases between the nuclei. This make the nuclei more 'exposed' to each other than they are when 'shielded' by the electrons b/w them. This causes repulsion.)

Please correct me if I am wrong

 You're mistaken about nuclei repulsion, IMHO. Because if it were the reason non-bonding electrons would contribute to repulsion also. The repulsion has quantum mechanics nature.

 The formula is: (one constructive interference) minus (one destructive interference) = Zero net interference (that's what stewie griffin has said)  

 

[Ligander]

This make the nuclei more 'exposed' to each other than they are when 'shielded' by the electrons b/w them. This causes repulsion

Mutual repulsion of nuclei exerts influence upon length of bond and energy of bond but not upon the bond order.