Group 16 Elements — Revision Notes

⚡ 30-Second Revision

Elements — O, S, Se, Te, Po (Chalcogens)
Valence e- config — $ns^2 np^4$
Common O.S. — $-2$ (O), $-2, +2, +4, +6$ (S, Se, Te, Po)
Trends Down Group

- Atomic/Ionic size: Increases - Ionization Enthalpy: Decreases - Electronegativity: Decreases - Metallic Character: Increases - Acidic character of $H_2E$: Increases ($H_2O < H_2S < H_2Se < H_2Te$) - Thermal stability of $H_2E$: Decreases ($H_2O > H_2S > H_2Se > H_2Te$) - Reducing character of $H_2E$: Increases ($H_2O < H_2S < H_2Se < H_2Te$) - Bond angle in $H_2E$: Decreases ($H_2O (104.5^circ) > H_2S (92.1^circ)$)

Anomalous O — Small size, high EN, no d-orbitals. Leads to H-bonding in $H_2O$, max covalency 4, less negative electron gain enthalpy than S.
Allotropes — O ($O_2, O_3$), S (Rhombic $S_8$, Monoclinic $S_8$, Plastic).
Key Compounds — $OF_2$ (O is $+2$), $SF_6$ (stable, octahedral), $SF_4$ (see-saw).

2-Minute Revision

Group 16 elements, or chalcogens, are O, S, Se, Te, Po, all having $ns^2 np^4$ valence electrons. Their primary oxidation state is $-2$, but S, Se, Te, Po can also show $+2, +4, +6$ due to vacant d-orbitals.

Down the group, atomic size and metallic character increase, while ionization enthalpy and electronegativity decrease. Oxygen is anomalous due to its small size, high electronegativity, and lack of d-orbitals.

This leads to hydrogen bonding in water (making it a liquid) and a less negative electron gain enthalpy than sulfur. Hydrides ($H_2E$) show increasing acidic and reducing character but decreasing thermal stability and bond angles down the group.

Allotropy is common: oxygen forms $O_2$ and $O_3$, while sulfur forms rhombic ($S_8$), monoclinic ($S_8$), and plastic forms. $SF_6$ is exceptionally stable due to steric protection. Remember the inert pair effect, which explains the increasing stability of the $+4$ oxidation state for heavier elements like Te and Po, at the expense of the $+6$ state.

5-Minute Revision

Let's quickly review the essentials of Group 16 elements, the chalcogens. This family includes Oxygen (O), Sulfur (S), Selenium (Se), Tellurium (Te), and Polonium (Po). Their defining feature is the $ns^2 np^4$ valence electron configuration, meaning they have six electrons in their outermost shell.

This drives their tendency to gain two electrons, resulting in a common $-2$ oxidation state. However, elements from sulfur onwards possess vacant d-orbitals, allowing them to expand their octet and exhibit positive oxidation states like $+2, +4$, and $+6$.

For example, sulfur in $SF_4$ is $+4$, and in $SF_6$ it's $+6$. Oxygen, lacking d-orbitals, is limited to a maximum covalency of four and primarily shows $-2$, except in $OF_2$ where it is $+2$.

Key periodic trends are crucial: As you move down the group, atomic and ionic radii increase, while ionization enthalpy and electronegativity decrease. This leads to an increase in metallic character from non-metals (O, S) to metalloids (Se, Te) and finally a metal (Po). The electron gain enthalpy generally becomes less negative down the group, but remember the anomaly: oxygen has a less negative electron gain enthalpy than sulfur due to inter-electronic repulsion in its compact $2p$ subshell.

Oxygen exhibits significant anomalous behavior due to its small size, high electronegativity, and absence of d-orbitals. This explains why water ($H_2O$) is a liquid (due to hydrogen bonding) while $H_2S$ is a gas, and why oxygen forms $O_2$ (double bond) while sulfur forms $S_8$ rings (single bonds).

Consider the hydrides ($H_2E$): Their acidic character ($H_2O < H_2S < H_2Se < H_2Te$) and reducing character ($H_2O < H_2S < H_2Se < H_2Te$) increase down the group, while thermal stability ($H_2O > H_2S > H_2Se > H_2Te$) and bond angles ($H_2O (104.5^circ) > H_2S (92.1^circ)$) decrease. The decreasing bond strength (E-H) is the underlying reason for these trends.

Allotropy is also important: Oxygen exists as dioxygen ($O_2$) and ozone ($O_3$). Sulfur has multiple forms, notably rhombic ($S_8$, stable below $369,\text{K}$) and monoclinic ($S_8$, stable above $369,\text{K}$), and plastic sulfur (polymeric chains).

Finally, remember the exceptional stability of $SF_6$ due to steric protection of the sulfur atom by fluorine, preventing hydrolysis. The inert pair effect explains why $+4$ oxidation states become more stable than $+6$ for heavier elements like Te and Po.

Prelims Revision Notes

Group 16 Elements (Chalcogens)

1. General Characteristics:

Elements: — Oxygen (O), Sulfur (S), Selenium (Se), Tellurium (Te), Polonium (Po).
Valence Electron Configuration: — $ns^2 np^4$ (6 valence electrons).
Oxidation States:

* Commonly $-2$. Tendency to show $-2$ decreases down the group. * S, Se, Te, Po can show $+2, +4, +6$ due to vacant d-orbitals. * Stability of $+6$ O.S. decreases down the group (e.g., $SF_6$ stable, $TeF_6$ less stable). * Stability of $+4$ O.S. increases down the group (inert pair effect). * Oxygen: Primarily $-2$. Exceptions: $-1$ (peroxides, $H_2O_2$), $-1/2$ (superoxides, $KO_2$), $+2$ (in $OF_2$).

2. Atomic and Physical Properties Trends (Down the Group):

Atomic/Ionic Radii: — Increase (due to new shells).
Ionization Enthalpy: — Decreases (larger size, less attraction).
Electron Gain Enthalpy: — Becomes less negative (less exothermic). Anomaly: Oxygen has less negative EGE than Sulfur (due to small size, inter-electronic repulsion).
Electronegativity: — Decreases (O is 2nd most EN after F).
Metallic Character: — Increases (O, S non-metals; Se, Te metalloids; Po metal).

3. Anomalous Behavior of Oxygen:

Reasons: — Small size, high electronegativity, absence of d-orbitals.
Consequences:

* Forms $O_2$ (pπ-pπ double bond), others form single bonds ($S_8$). * Max covalency of 4 (e.g., $H_3O^+$), others up to 6 ($SF_6$). * Hydrogen bonding (e.g., $H_2O$ is liquid, $H_2S$ is gas). * Less negative electron gain enthalpy than sulfur.

4. Hydrides ($H_2E$):

General Formula: — $H_2E$.
Physical State: — $H_2O$ is liquid (H-bonding), others are gases.
Acidic Character: — Increases down the group ($H_2O < H_2S < H_2Se < H_2Te$). (Weaker E-H bond).
Thermal Stability: — Decreases down the group ($H_2O > H_2S > H_2Se > H_2Te$). (Weaker E-H bond).
Reducing Character: — Increases down the group ($H_2O < H_2S < H_2Se < H_2Te$). (Easier H-donation).
Bond Angle: — Decreases down the group ($H_2O (104.5^circ) > H_2S (92.1^circ) > H_2Se (91^circ) > H_2Te (90^circ)$).

5. Halides:

Dihalides ($EX_2$) — $SCl_2, SeCl_2$.
Tetrahalides ($EX_4$) — $SF_4$ (see-saw geometry), $SeF_4, TeF_4$. Stability of $+4$ increases down group.
Hexahalides ($EX_6$) — $SF_6, SeF_6, TeF_6$. Only with F. $SF_6$ is exceptionally stable (steric protection, kinetic inertness). Octahedral geometry.
Oxygen Halides — $OF_2$ (O is $+2$), $O_2F_2$.

6. Allotropy:

Oxygen: — Dioxygen ($O_2$), Ozone ($O_3$, pale blue, pungent, strong oxidizing agent).
Sulfur: — Rhombic ($alpha$-sulfur, $S_8$ rings, stable below $369,\text{K}$, yellow), Monoclinic ($\beta$-sulfur, $S_8$ rings, stable above $369,\text{K}$, pale yellow), Plastic (amorphous, polymeric chains).

Vyyuha Quick Recall

To remember Group 16 elements: Old Students Select Tellurium Polonium. (Oxygen, Sulfur, Selenium, Tellurium, Polonium)