Question

Find the oxidation number of $Cr$in ${K_2}C{r_2}{O_7}$?

Answer 1

Take $x$ as the oxidation number of $Cr$, $K$will have an oxidation number of group $1$ elements and oxygen will have its general oxidation number. Add them all and multiply each of the oxidation numbers with their respective subscripts. The Sum of all the oxidation numbers will be zero.

Complete answer:
The oxidation number of an atom could be positive, negative, or zero. The atom with higher electronegativity, typically a nonmetallic element, is assigned a negative oxidation number, while metallic elements are typically assigned positive oxidation numbers.
There are certain rules for finding the oxidation number of an atom. They are as follows:
Elements in their pure form have zero oxidation numbers.
The oxidation number of an ion is the same as its charge
The oxidation number of metals is of group one is $+ 1$and of group two is $+ 2$
Hydrogen has two possibilities of oxidation number $+ 1$ and$- 1$. $+ 1$ is when it is bonded to a non-metal. And $- 1$ is when it is bonded to a metal.
Oxygen can have$- 2$, $+ 2$ and $- 1$oxidation states. Generally it has $- 2$ oxidation number, but in peroxides the oxidation number of oxygen becomes $- 1$and in oxygen difluoride oxygen has $+ 2$ oxidation state.
Halogens have oxidation numbers as $- 1$ except when they are combined with oxygen or fluorine.
The sum of the oxidation numbers in a neutral compound is zero
Now let us calculate the oxidation number of $Cr$in ${K_2}C{r_2}{O_7}$
${K_2}C{r_2}{O_7}$
$K$is a group element so it will have $+ 1$oxidation number, oxygen will have oxidation number $- 2$ and let $x$ be the oxidation number of $Cr$, and the compound is neutral so their sum will be zero.
Thus ${K_2}C{r_2}{O_7} = 0$
$\Rightarrow 2( + 1) + 2x + 7( - 2) = 0$
$\Rightarrow 2 + 2x - 14 = 0$
$\Rightarrow 2x - 12 = 0$
$\Rightarrow 2x = 12$
$\Rightarrow x = \dfrac{{12}}{2}$
$\Rightarrow x = + 6$
Therefore the oxidation number of $Cr$in ${K_2}C{r_2}{O_7}$ is $+ 6$.

Note:
Oxidation number is a number that is assigned to an atom in a substance. It indicates if electrons are lost or gained. If the oxidation number is positive, then this means that the atom loses electrons. This happens when an atom undergoes oxidation. If an atom has a negative oxidation number, it means the atom gains electrons. This is the case when an atom undergoes reduction. And if the oxidation number of the atom is zero, then the atom neither gains nor loses electrons.