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The oxidation number of the underlined atom in the following species: (1) $H\underline{Au}Cl_4$ is +3 (2) $\underline{Cu}_2O$ is $-1$ (3) $\underline{Cl}O_3^-$ is +5 (4) $K_2\underline{Cr}_2O_7$ is +6. Identify the incorrect option.
A
$HAuCl_4$ is +3
B
$Cu_2O$ is $-1$
C
$ClO_3^-$ is +5
D
$K_2Cr_2O_7$ is +6
Detailed Solution
In $Cu_2O$, oxygen is $-2$; for charge balance, $2x+(-2)=0\Rightarrow x=+1$. So copper in $Cu_2O$ is $+1$, not $-1$ — the statement in option (2) is incorrect. (Options 1, 3, 4 are correctly assigned oxidation states.)