sigma and pi bonds
/ SIG-ma and PY /
When two atoms share electrons, the overlap of their orbitals can happen in two distinct geometries, and these give the two basic kinds of covalent bond. A sigma bond forms when orbitals overlap head-on, straight along the line joining the two nuclei, putting electron density right between them. A pi bond forms when two p orbitals overlap sideways, above and below that line, like two parallel sausages of electron density. Together they explain single, double and triple bonds.
A single bond is always one sigma bond — strong, because the head-on overlap is large, and free to rotate, because the bond is symmetric about the axis. A double bond is one sigma plus one pi bond; a triple bond is one sigma plus two pi bonds. Pi bonds are generally weaker than sigma bonds (sideways overlap is less effective) and they lock the molecule rigid, because rotating the bond would tear the parallel p orbitals apart. So an N2 triple bond is one sigma and two pi bonds, shorter and far stronger than a single N-N bond.
The sigma/pi distinction is the engine behind bond order, bond length and bond strength, and it shows up everywhere in inorganic chemistry. Pi bonding explains why multiple bonds are short and stiff, why nitrogen prefers N2 (a robust triple bond) while phosphorus prefers P4 (single bonds, because heavy atoms make poor pi bonds), and pi interactions are central to metal-ligand bonding, where pi-acceptor ligands like CO use empty pi orbitals to accept electron density back from a metal.
In N2, the two nitrogens are held by one sigma bond (head-on overlap of orbitals pointing at each other) and two pi bonds (sideways overlap of the perpendicular p orbitals). Three bonds, short and very strong, is why dinitrogen is so unreactive.
The N2 triple bond: one sigma plus two pi bonds.
A single bond is always sigma, never pi; pi bonds only ever come on top of a sigma bond. So you never get a pi bond by itself between two atoms, which is why rotating a double bond (breaking the pi part) costs energy.