Q.Assertion: The highest oxidation state of osmium is +8.
Reason: Osmium is a 5d-block element.

The key idea is that the ability to reach a +8 oxidation state depends on the availability of all 5d and 6s electrons for bonding, not simply on being a 5d element.

Reasoning:

1. Osmium (Os, atomic number 76) has the electronic configuration $[Xe]\,4f^{14}\,5d^{6}\,6s^{2}$. It can use all eight valence electrons (six from 5d and two from 6s) to form compounds like $\ce{OsO4}$, giving a +8 state. …

The key here is to separate two ideas: what is true from why it is true. The assertion and reason are both factually correct, but the reason given is too vague to be the cause of the assertion.

Let’s break it down.

1. Check the assertion: Is +8 the highest oxidation state of osmium?

   Yes. Osmium tetroxide, OsO₄, is a well-known compound where osmium is in the +8 oxidation state. This is the highest oxidation state known for any element in the periodic table (shared with ruthenium in RuO₄ and xenon in XeO₄). So the assertion is true.

2. Check the reason: Is osmium a 5d-block element?

   Osmium (atomic number 76) has the electronic configuration $[Xe]\,4f^{14}\,5d^{6}\,6s^{2}$. It lies in the third row of the d-block, which is the 5d series. So the reason is also true.

3. Now the critical question: Does being a 5d element explain the +8 oxidation state? …

Concept: Oxidation States in d-Block Elements

The highest oxidation state of a transition metal is determined by the number of electrons available in the outermost s and d orbitals that can be lost or shared, not merely by being a 5d-block element.

Method: Electronic Configuration Analysis

Step 1: Write the electronic configuration of osmium (Os, atomic number 76).

• Os: $[Xe]\, 4f^{14}\, 5d^{6}\, 6s^{2}$

Step 2: Identify the total number of electrons in the valence shell (6s + 5d).

• $6s^2 + 5d^6 = 8$ valence electrons

Step 3: Determine the maximum possible oxidation state.

• All 8 valence electrons can be lost or shared, giving a maximum oxidation state of +8 (e.g., in $\text{OsO}_4$).

Step 4: Evaluate the reason. …

Here’s a breakdown of the common mistakes students make on this question and how to avoid each.

Mistake 1: Confusing “Highest Oxidation State” with “Most Common Oxidation State”

• What students do wrong: They think that because Os is a 5d element, it must show +8. Or they confuse the highest possible state with the most stable state (e.g., Os in +4 or +6 is more common in compounds).
• Why it’s wrong: The assertion is about the maximum oxidation state, not the most frequent one. Osmium does reach +8 in compounds like OsO₄. The reason (5d-block) is true but doesn’t explain why +8 is possible — that depends on the number of valence electrons available (Os has 8 valence electrons: 5d⁶ 6s²).
• How to avoid: Always distinguish between “highest possible” and “most common.” For d-block elements, the highest oxidation state often equals the group number (Os is in Group 8), but the reason must link to electron configuration, not just block position.

Mistake 2: Thinking the Reason is the Correct Explanation

• What students do wrong: They see both statements are true and tick option (i) without checking if the reason causally explains the assertion.
• Why it’s wrong: Being a 5d-block element is a property, not a mechanism. The real explanation is that Os has 8 valence electrons (5d⁶ 6s²) and can lose all of them to achieve +8. Many 5d elements (like Pt, Au) do not show +8.
• How to avoid: For assertion-reason questions, ask: “Does the reason directly cause the assertion?” If not, it’s option (ii). Memorise that for Os, the +8 state is due to its electronic configuration (n-1)d⁶ ns², not just being in the 5d series.

Mistake 3: Forgetting that Osmium is in Group 8

• What students do wrong: They think the highest oxidation state of any d-block element is less than +8, or they confuse Os with other 5d elements like Pt or Au.
• Why it’s wrong: Os is in Group 8, and its maximum oxidation state is +8 (as in OsO₄). This is a unique fact — only Os and Ru (Group 8) show +8. …