  Usually, the octet rule has limited the 2D materials by elements within
groups IIIA_VA, whose outmost electrons can be considered to form hybridization
orbit by s wave and p wave. The hybridization orbits can accommodate all the
outmost electrons and form robust sigma bonds. Over VA group, the outmost
electrons seem too abundant to be utilized in hybridization orbits. Here we
show a spd2 hybridization rule, accommodating all the outmost electron of
halogen elements. Each atom can be connected with contiguous atom by robust
sigma bond, and carries one dangling lone-unpaired electron, implying formation
of pi bond is possible. One atomic iodine layer can be robustly locked by the
sigma bond, forming iodiene sheet by spd2 hybridization orbits. With
application of compression strain, the pi bond forms, and the band inversion
occurs simultaneously at the valence band and conductance band. Dirac points
(line) and topological nontrivial points (arc or hoop) suggests the
transformation of Dirac semimetal or topological semimetal happens.
