Question

Oxidation number of osmium (Os) in $Os{{O}_{4}}$ is:
(a)- +8
(b)- +6
(c)- +7
(d)- +4

Answer 1

Hint: Oxidation number is the charge of the atom when it is present in a combined state. There is no charge on the overall molecule. The oxidation number of oxygen is -2 in $Os{{O}_{4}}$. So, the oxidation number of osmium can be calculated.

Complete answer:
The oxidation number of an element may be defined as the charge in which an atom of the element has its ion or appears to have when present in the combined state with other atoms. The oxidation number is also called the oxidation state.
 Osmium is the element of d-block. The transition elements have their valence electron in two different sets of orbitals, i.e., (n-1)d and ns. Since, there is very little difference in the energies of these orbitals, both energy levels can be used for bond formation. In some compounds, we may presume that the two electrons from the ns orbital of a transition element are used to give an oxidation state of +2 and the (n-1)d electrons remain unaffected. But the higher oxidation states like +3, +4, +5, +6, +7, and +8 correspond to the ns and (n-1)d electrons in the transitions series of elements.
In $Os{{O}_{4}}$, osmium is bonded to four oxygen atoms each with a double bond. So, the oxygen has a -2 oxidation state.

So, the oxidation state of osmium can be calculated as:
$x+4(-2)=0$
$x=+8$
Hence the oxidation number of osmium is +8.

So, the correct answer is an option (a)- +8.

Note:
There is no charge on the molecule hence the sum of the oxidation state of the charge is equal to zero. If there is a charge present on the overall molecule, the sum of the oxidation state of atoms is equal to the charge of the molecule.