For the reaction, ${\mathrm{CrO}}_4^{-2}+?⟶{\mathrm{Cr}}_2{\mathrm{O}}_7^{-2}$ the missing ion is

The reaction is given.

             ${\mathrm{CrO}}_4^{2-}+{\mathrm{H}}^{+}\to {\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$

An ion is an atom or molecule that has a net electrical charge. The charge of an electron is believed to be negative by convention, and this charge is equal to and opposite to the charge of a proton, which is thought to be positive by convention.

${\mathrm{H}}^{+}$ is the missing ion.