Question

What is the oxidation number of underlined element?

$K\underset{–}{Cr}{O}_{3}Cl$

Answer 1

Let $x$ be the oxidation number of $Cr$ in $K\underset{–}{Cr}{O}_{3}Cl$.
Since the overall charge on the complex is $0$, the sum of oxidation states of all elements in it should be equal to $0$.
Therefore, $+1+x+3(-2)+(-1)=0$
$x=+6$
Hence, the oxidation number of Cr in $K\underset{–}{Cr}{O}_{3}Cl$ is $+6$.