Assertion (A): The highest oxidation state of osmium is +8 .
Reason (R): Osmium is a 5d-block element.
1. Both (A) and (R) are true and (R) is the correct explanation of (A).
2. Both (A) and (R) are true but (R) is not the correct explanation of (A).
3. (A) is true but (R) is false.
4. Both (A) and (R) are false.

Hint: Osmium is 5th period element
Generally, within the transition series, the highest oxidation state increases with the increase of atomic number, reaching to a
maximum in the middle and then starts decreasing.
The highest oxidation state of osmium is +8 due to its ability to expand its octet by using it's all 8 electrons (2 from 6s and 6
from 5d). Hence, both assertion and reason are true but the reason is not the correct explanation of assertion.