antibonding orbital
/ AN-tee-BOND-ing OR-bih-tal /
Go back to the two pond ripples, but now let them meet out of step — a crest arriving where the other has a trough. They cancel, and right between them the water goes flat and still. Electron waves can cancel the same way. When two atomic orbitals combine out of phase, the electron density vanishes in a plane between the nuclei. A molecular orbital built like this is an antibonding orbital, and it does the opposite of holding the atoms together.
Mathematically it comes from subtracting one atomic orbital from the other, (A - B). That subtraction leaves a node — a surface of exactly zero electron density — sitting between the two nuclei. With little or no negative charge in the middle to shield them, the positive nuclei feel each other's repulsion more strongly, so an antibonding orbital lies higher in energy than the atomic orbitals it came from. Chemists mark these orbitals with an asterisk, like sigma-star or pi-star, to flag that they push atoms apart. An antibonding orbital is destabilized by exactly a bit more than the matching bonding orbital is stabilized.
Antibonding orbitals are not just bookkeeping; their occupation is what weakens or breaks bonds. He2 fails to exist because its four electrons fill one bonding and one antibonding orbital, exactly canceling. In O2, two electrons sit in pi-star antibonding orbitals, which is why oxygen's bond is weaker than a triple bond and why it is magnetic. Filling antibonding orbitals (for example by absorbing light) is also a common first step in a molecule reacting or falling apart.
Helium refuses to form He2 because of antibonding orbitals: its four electrons fill the bonding sigma and the antibonding sigma-star equally. Bond order is (2 - 2)/2 = 0, so there is no net bond — the antibonding pair cancels the bonding pair.
Antibonding orbitals have a node between the nuclei and sit above the parent atomic orbitals.
An antibonding orbital is raised slightly more than its bonding partner is lowered; that small asymmetry is why fully filling both a bonding and an antibonding orbital leaves the pair of atoms a touch worse off than apart.