Electronic Configuration, Oxidation States — Revision Notes

⚡ 30-Second Revision

Group 16 Valence Config: — $ns^2np^4$ (6 valence electrons)
Oxygen (O): — $2s^22p^4$. No d-orbitals. Max covalency 2.

- Oxidation States: -2 (most common), -1 (peroxides, e.g., $\text{H}_2\text{O}_2$), -1/2 (superoxides, e.g., $\text{KO}_2$), +2 (with F, e.g., $\text{OF}_2$).

Sulfur (S), Selenium (Se), Tellurium (Te): — Have vacant d-orbitals.

- Oxidation States: -2, +2, +4, +6 (due to d-orbital expansion).

Polonium (Po): — Significant inert pair effect.

- Oxidation States: +2 (most stable), +4, +6 (less stable).

Inert Pair Effect: — $ns^2$ electrons become reluctant to bond for heavier elements, stabilizing lower oxidation states.
d-orbital Expansion: — Allows elements from period 3 onwards to expand octet and show higher positive oxidation states.

2-Minute Revision

Group 16 elements, or chalcogens, all share the valence electronic configuration $ns^2np^4$, meaning they have six valence electrons. This configuration drives their primary tendency to gain two electrons, leading to a common -2 oxidation state.

However, there are crucial differences down the group. Oxygen, being the first member, is highly electronegative and lacks vacant d-orbitals. This restricts its oxidation states mainly to negative values (-2, -1 in peroxides, -1/2 in superoxides) and only +2 when bonded to the more electronegative fluorine.

It cannot expand its octet. In contrast, sulfur, selenium, and tellurium possess vacant d-orbitals. This allows them to promote electrons and expand their octet, enabling them to exhibit positive oxidation states of +2, +4, and +6, particularly with highly electronegative elements like oxygen and fluorine.

As we move further down to polonium, the inert pair effect becomes significant, where the $ns^2$ electrons become less involved in bonding, making the +2 oxidation state more stable than the higher +4 or +6 states.

Remember these trends and the underlying electronic reasons for NEET.

5-Minute Revision

A thorough understanding of electronic configuration and oxidation states for Group 16 elements is vital for NEET. All Group 16 elements have a general valence electronic configuration of $ns^2np^4$. This means they have six valence electrons, and their primary tendency is to gain two electrons to achieve a stable octet, resulting in a common -2 oxidation state. For example, in $\text{H}_2\text{S}$, sulfur is -2.

Oxygen (O): The first member, oxygen ($2s^22p^4$), is unique. It's highly electronegative and lacks vacant d-orbitals in its second shell. This prevents it from expanding its octet. Consequently, its most common oxidation state is -2 (e.

g., $\text{H}_2\text{O}$). It can also show -1 in peroxides (e.g., $\text{H}_2\text{O}_2$), -1/2 in superoxides (e.g., $\text{KO}_2$), and uniquely, +2 with fluorine (e.g., $\text{OF}_2$) because fluorine is more electronegative.

It cannot achieve +4 or +6 states.

Sulfur (S), Selenium (Se), Tellurium (Te): These elements, from the third period onwards, have vacant d-orbitals (3d for S, 4d for Se, 5d for Te). This allows them to promote their $s$ and $p$ electrons to these d-orbitals, leading to an expansion of their octet. This d-orbital expansion enables them to form more bonds and exhibit positive oxidation states of +2, +4, and +6. For instance, sulfur forms $\text{SF}_2$ (+2), $\text{SF}_4$ (+4), and $\text{SF}_6$ (+6).

Polonium (Po): For the heaviest element, Polonium ($6s^26p^4$), the inert pair effect becomes significant. The $6s^2$ electrons become increasingly reluctant to participate in bonding due to poor shielding by inner d and f electrons. This stabilizes the lower oxidation states. Thus, for Polonium, the +2 oxidation state is generally more stable than +4, and +6 is very rare. For example, $\text{PoCl}_2$ is more stable than $\text{PoCl}_4$.

Key Takeaways:

1
Oxygen's uniqueness: — No d-orbitals, high electronegativity, limited positive states.
2
d-orbital expansion: — Explains +4, +6 for S, Se, Te.
3
Inert pair effect: — Explains stability of +2 for Po.

Practice calculating oxidation states in various compounds, especially those involving oxygen exceptions, and understand the reasons behind the trends down the group.

Prelims Revision Notes

1
General Electronic Configuration: — All Group 16 elements (Chalcogens) have a valence shell electronic configuration of $ns^2np^4$. This means they possess 6 valence electrons.
2
Common Oxidation State: — The most common and stable oxidation state for Group 16 elements is -2, achieved by gaining two electrons to complete their octet ($ns^2np^6$).
3
Oxygen's Unique Behavior:

* Electronegativity: Oxygen is the second most electronegative element (after fluorine). * d-orbitals: It lacks vacant d-orbitals in its valence shell (n=2). * Oxidation States: Primarily -2 (e.g., $\text{H}_2\text{O}$). Also -1 in peroxides (e.g., $\text{H}_2\text{O}_2$), -1/2 in superoxides (e.g., $\text{KO}_2$), and +2 only with fluorine (e.g., $\text{OF}_2$). * Covalency: Maximum covalency is 2 (cannot expand octet).

1
Sulfur, Selenium, Tellurium:

* d-orbitals: Possess vacant d-orbitals in their valence shells (3d for S, 4d for Se, 5d for Te). * d-orbital Expansion: This allows them to promote electrons and expand their octet, forming more than two bonds. * Positive Oxidation States: Can exhibit +2, +4, and +6 oxidation states (e.g., $\text{SF}_2, \text{SF}_4, \text{SF}_6$).

1
Polonium:

* Inert Pair Effect: Significant for Polonium. The $6s^2$ electrons become reluctant to participate in bonding. * Oxidation States: +2 becomes the most stable oxidation state, while +4 and +6 are less stable and less common.

1
Trends Down the Group:

* Electronegativity: Decreases from O to Po. * Metallic Character: Increases from O (non-metal) to Po (metal/metalloid). * Stability of +6 Oxidation State: Decreases down the group (most stable for S, least for Po). * Stability of +2 Oxidation State: Increases down the group (due to inert pair effect).

1
Calculation of Oxidation States: — Remember that the sum of oxidation states in a neutral compound is zero, and in an ion, it equals the charge of the ion. Assign known values (e.g., H=+1, F=-1, Group 1 metals=+1, Group 2 metals=+2, O=-2 typically) to find the unknown.

Vyyuha Quick Recall

Only Strong Students Try Positive Levels (for Group 16 elements: Oxygen, Sulfur, Selenium, Tellurium, Polonium, Livermorium).

For oxidation states: Oxygen: Negative Only (mostly -2, but +2 with F) Sulfur: Six Possibilities (+2, +4, +6 due to d-orbitals) Polonium: Preferred Two (due to inert pair effect, +2 is most stable)