Stereoisomerism

In a nutshell

Stereoisomers are molecules with the same structural formula but the atoms are arranged differently in space. E/Z isomerism is a type of stereoisomerism. CIP rules can be used to determine whether an isomer is E or Z isomer. Cis-trans isomerism is a sub-category of E/Z isomerism.

Double bonds

$C=C$ bonds and the atoms directly attached to these carbon atoms are arranged in a trigonal planar shape, with a bond angle of $120 \degree C$ .