  Metallization and dissociation are key transformations in diatomic molecules
at high densities particularly significant for modeling giant planets. Using
X-ray absorption spectroscopy and atomistic modeling, we demonstrate that in
halogens, the formation of a \textit{connected} molecular structure takes place
at pressures well below metallization. Here we show that the iodine diatomic
molecule first elongates of $\sim$0.007 \AA~up to a critical pressure of $P_c$
$\backsim$7~GPa developing bonds between molecules. Then its length
continuously decreases with pressure up to 15-20~GPa. Universal trends in
halogens are shown and allow to predict for chlorine a pressure of 42$\pm$8~GPa
for molecular bond-length reversal. Our findings tackle the molecule
invariability paradigm in diatomic molecular phases at high pressures and may
be generalized to other abundant diatomic molecules in the universe, including
hydrogen.
