delocalized pi system
/ dee-LOH-kuh-lyzd pie SIS-tem /
Two atoms are the easy case, but molecular orbital theory really earns its keep with three or more atoms in a row. When several sideways-overlapping p orbitals line up along a chain or ring, their electrons do not stay trapped between any one pair of atoms. Instead they smear out over the whole framework. A delocalized pi system is that shared, spread-out cloud of pi electrons running across several atoms at once.
Picture three or more p orbitals standing side by side, each like a dumbbell pointing perpendicular to the molecular plane. Combining them by LCAO gives the same number of pi molecular orbitals, ranging from a fully bonding one (all lobes in phase, electron density flowing smoothly across the whole chain) up to a fully antibonding one (sign flipping at every atom, with many nodes). The electrons fill these from the bottom, and the lowest, most-bonding orbitals genuinely belong to all the atoms together — not to individual bonds. This is what the older idea of resonance was groping toward: resonance draws several Lewis structures and tells you the truth is a blend, while MO theory just draws one delocalized orbital that is the blend.
Delocalization is everywhere in real chemistry and it has real consequences. It is why the carbonate ion CO3 2-minus has three identical bonds rather than one double and two single, why benzene is a flat, unusually stable ring with all six carbon-carbon bonds equal, and why nitrate and ozone behave as they do. Spreading electrons out lowers their energy, which is the extra stability chemists call delocalization or resonance energy. And because delocalization shrinks the gap between the highest filled and lowest empty pi orbitals, extended pi systems are exactly the molecules that absorb visible light and give us dyes and pigments their color.
In the carbonate ion, CO3 2-minus, a Lewis structure must draw one C=O double bond and two C-O single bonds — but the three bonds are actually identical. MO theory explains it cleanly: the pi electrons occupy a single delocalized orbital spread evenly over all three oxygens, so all three bonds are the same.
Delocalized pi orbitals spread electrons over many atoms — one orbital does what several resonance structures only hint at.
Delocalization and resonance describe the same physical reality from two angles: resonance is a patch on Lewis structures, while a delocalized MO is the more direct picture — neither means electrons are 'flipping' between structures.