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Alkenes contain at least one C=C double bond consisting of a sigma bond and a pi bond; the pi bond restricts rotation, locking the geometry around the double bond. This restricted rotation enables cis/trans geometric isomerism, more precisely described by the E/Z system using CIP priority rules: E (entgegen, 'opposite') when higher-priority groups are on opposite sides, Z (zusammen, 'together') when on the same side. Alkene carbons are sp2 hybridized with planar trigonal geometry. The electron-rich pi bond is the site of reactivity in nearly all alkene reactions.
Practice E/Z assignment starting with disubstituted alkenes, then tetrasubstituted. Confirm CIP rankings using explicit atomic-number comparisons. Connect the planar geometry to why cis/trans isomers have different physical properties.
When you learned to name alkanes using IUPAC rules, carbon chains were flexible — single bonds allow free rotation, so an alkane can adopt countless conformations that interconvert freely at room temperature. Alkenes introduce a fundamental change in geometry: the C=C double bond consists of a sigma bond (end-on overlap, strong) and a pi bond (sideways overlap of adjacent p orbitals, weaker). That pi bond is the key to everything in alkene chemistry.
The p orbitals forming the pi bond must remain parallel for effective overlap. Rotating one carbon relative to the other would twist those orbitals out of alignment, breaking the pi bond — an energy cost of roughly 60 kcal/mol. This is far too large to overcome at room temperature. The consequence is that the two double-bond carbons are locked in a plane, and any substituents attached to them are frozen in space relative to each other. This is why cis-2-butene and trans-2-butene are two different compounds with different boiling points, not interconvertible conformations.
To name which isomer you have, chemists use the E/Z system based on CIP priority rules. For each double-bond carbon, you compare the two substituents using atomic number: the substituent whose first atom has the higher atomic number gets higher priority. If the higher-priority groups on each carbon are on the same side of the double bond, the isomer is Z (from German *zusammen*, "together"). If they are on opposite sides, it is E (*entgegen*, "opposite"). This system handles all cases — including trisubstituted alkenes where cis/trans is ambiguous — because CIP always produces a definite ranking as long as the two substituents on each carbon are different.
The sp2 hybridization of alkene carbons also determines the geometry around the double bond. Each sp2 carbon forms three bonds arranged at ~120° in a plane, with the remaining p orbital perpendicular to that plane. This means a double-bond carbon and all four atoms directly attached to it (two substituents plus the other alkene carbon) are coplanar. This planarity is exploited by the pi bond itself and has direct consequences for how reagents approach the alkene in reactions you will study next.
Finally, note that the pi bond is both the defining feature of alkene reactivity and the weaker of the two bonds in the C=C double bond. Bond dissociation energy data show the pi bond contributes roughly 60-65 kcal/mol on top of the sigma bond's ~90 kcal/mol. Reagents can selectively attack the pi bond without breaking the sigma bond — this is the basis of all electrophilic addition reactions. The electron-rich pi cloud acts as a nucleophile, attacking incoming electrophiles; the geometry of that pi system determines what faces are accessible and what stereochemical outcomes are possible.