hybridization
/ hy-brid-ih-ZAY-shun /
Imagine mixing two cans of paint, one blue and one yellow, to get a batch of identical green — the originals are gone, replaced by something new and uniform. Hybridization does the same with an atom's orbitals: it blends orbitals of different shapes and energies into a set of new, equivalent orbitals that point neatly toward the atoms a bond will reach.
Precisely, hybridization is the mixing of an atom's atomic orbitals (typically one s with some p orbitals) to form an equal number of hybrid orbitals with new shapes and directions. Mixing s with one p gives two sp hybrids (linear, 180°); with two p gives three sp² hybrids (trigonal, 120°); with three p gives four sp³ hybrids (tetrahedral, 109.5°). The hybrids are what actually overlap to make sigma bonds.
It matters because plain s and p orbitals point in the wrong directions to explain observed molecular shapes; hybridization is the bookkeeping device that lines them up. The honest caveat: hybridization is a model, not a physical event — orbitals do not really 'mix' as a step in time. It is a convenient way to build localized bonds that match geometry, and you choose the hybridization to fit the shape, not the other way around.
In ethene (H₂C=CH₂), each carbon is sp² hybridized: three sp² hybrids form three sigma bonds in a flat triangle, while the leftover unhybridized p orbital points up and down and overlaps sideways to make the second, pi part of the double bond. That is why ethene is flat and cannot freely twist about the C=C bond.
sp² carbon: three flat sigma bonds plus one leftover p orbital for a pi bond.
A common confusion: the number after sp counts p orbitals mixed in, not bonds. sp³ means one s plus three p (four hybrids total); the '3' is not the number of bonds. Also, only orbitals on the same atom hybridize — you never hybridize orbitals from two different atoms.