answer 1
check out the number of oxygens on the right hand side = 7 so there must be 7 on the left hand side!
check out the number of chromiums (atoms/ions) on the right hand side = 2 therefore there must be two on the lhs
now check out the electrical charges on the right han side compared to the lhs ... you need two negatives on the lhs
hence Cr2O7(2-)
this is the standard reduction of the dichromate ion half equation (orange to green)
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