Question

The oxidation state of $Cr$ in ${K_2}C{r_2}{O_7}$ is
A. +7
B. +6
C. +3
D. +2

Answer 1

Oxidation number can be described as number allocated to elements in a chemical combination. The oxidation number is basically the count of electrons that atoms in a molecule can share, lose or gain while forming chemical bonds with atoms of a different element.

Complete step by step answer: Oxidation number is referred to as oxidation state. Oxidation number of an atom is defined as the charge that an atom has on forming ionic bonds with other heteroatoms. An atom having higher Electronegativity is given a negative oxidation state, even in covalent bonds. Whereas atoms having lower Electronegativity are given positive oxidation states.
Oxidation number or state of an atom/ion is the number of electrons of an atom/ion that molecule has either gained or lost compared to the neutral atom.
Electropositive metal atoms, of group 1, 2, and 3 lose a specific number of electrons and always have constant positive oxidation numbers. While group 17, 16 shows negative oxidation state.
Oxidation number of an atom or ion in a molecule is assigned by summing up the constant oxidation state of other atoms/molecules/ions that are bonded to it and equating the total oxidation state of a molecule or ion to the total charge of the molecule/ion.
Here in this case we have ${K_2}C{r_2}{O_7}$ and we have to calculate the oxidation state of $Cr$.
The overall charge of this molecule = 0
We know that the oxidation state of oxygen = -2
And oxidation state of potassium = +1
So total charge on the molecule = 2(O.S. of potassium) + 2(O. S. of chromium) + 7(O. S. of oxygen)
$0 = 2( + 1) + 2($ Oxidation state of $Cr) + 7( - 2)$
$0 = 2 + 2($ Oxidation state of $Cr) - 14$
On solving this equation we will get the oxidation state of chromium as
$2($Oxidation state of $Cr) = 12$
Oxidation state of chromium = +6

Hence the correct option will be B.

Note: Oxidation state of elemental atoms such as sodium, magnesium, iron is zero. Also the oxidation state of neutral molecules such as nitrogen, water, etc is zero. And the oxidation state of homopolar molecules is also zero