Fill in the blanks:
$(1)$ In the dichromate ion,the $O-Cr-O$ bond angle is ....... $^o$.
$(2)$ The reaction $3Sn^{2+} \to 3Sn^{4+} + 6e^-$ occurs via .........,in which $Sn^{2+}$ is ......... .
$(3)$ The symbols for stannous and stannic are ......... and ......... respectively.
$(4)$ $V_2O_4$ dissolves in acidic compounds to give ......... salts.

(N/A) $(1)$ The $O-Cr-O$ bond angle in the dichromate ion $(Cr_2O_7^{2-})$ is $126^o$.
$(2)$ The reaction $3Sn^{2+} \to 3Sn^{4+} + 6e^-$ occurs via oxidation,in which $Sn^{2+}$ is a reducing agent.
$(3)$ The symbols for stannous and stannic are $Sn^{2+}$ and $Sn^{4+}$ respectively.
$(4)$ $V_2O_4$ dissolves in acidic compounds to give vanadyl $(VO^{2+})$ salts.