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CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

Orbital overlap

Orbitals on two neighbouring atoms overlap when they occupy the same region of space. In-phase overlap builds electron density between the nuclei and forms a bond, and more overlap generally means a stronger bond; head-on overlap gives a σ bond, side-on overlap of parallel p orbitals a π bond.

Potential energy of two H atomsEnergy of two hydrogen atoms against the distance between their nuclei. Far apart the energy is zero; it falls to a minimum of −436 kJ/mol at 74 pm (the bond length; the depth is the bond energy) and rises steeply when the nuclei are pushed closer.74150225300−4360400distance between nuclei (pm)energy (kJ/mol)bond lengthbond energy

Energy falls as the orbitals overlap; the minimum sits at the bond length.

Example

As two H atoms approach, their 1s1s orbitals overlap more and the energy falls until nuclear repulsion takes over at 74 pm, the H–H bond length (bond energy 436 kJ/mol). Side-on p–p overlap is smaller than head-on overlap, which is why a π bond is weaker than a σ bond.

See alsoσ bond (sigma bond)π bond (pi bond)Valence bond theoryBond length