Group 16 Elements: The Oxygen Family | Chemca

P-Block Elements

Group 16 Elements: The Oxygen Family

By Chemca Editorial Team • Last Updated: January 2026 • 12 min read

Group 16 elements consist of Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te), and Polonium (Po). They are collectively known as Chalcogens (ore-forming elements).

1. General Trends & Properties

Electronic Configuration

The general valence shell electronic configuration is $ns^2 np^4$.

Atomic and Ionic Radii

Increases down the group due to the increase in the number of shells ($O < S < Se < Te < Po$).

Ionization Enthalpy

Decreases down the group. Group 16 elements have lower ionization enthalpy than Group 15 elements corresponding to the same period due to the extra stability of half-filled p-orbitals in Group 15.

Electron Gain Enthalpy

The Oxygen Anomaly

Oxygen has a less negative electron gain enthalpy than Sulphur. This is due to the compact nature of the oxygen atom, which results in stronger inter-electronic repulsion when an electron is added. From Sulphur onwards, the value becomes less negative down the group.
Order: $S > Se > Te > Po > O$ (in magnitude).

2. Chemical Properties

Oxidation States

-2: Common for Oxygen. Stability decreases down the group.
+2, +4, +6: Common for S, Se, Te. Oxygen generally does not show positive oxidation states (except with Fluorine, e.g., $OF_2$).
Inert Pair Effect: The stability of the +6 oxidation state decreases down the group, while +4 increases ($SF_6$ is very stable, but $TeF_6$ is an oxidizing agent).

Hydrides ($H_2E$)

Trends (from $H_2O$ to $H_2Te$):

Acidic Character: Increases ($H_2O < H_2S < H_2Se < H_2Te$). Bond dissociation energy decreases.
Thermal Stability: Decreases.
Boiling Point: $H_2S < H_2Se < H_2Te < H_2O$. Water has an exceptionally high boiling point due to Hydrogen Bonding.

3. Important Compounds

Dioxygen ($O_2$)

Prepared by heating chlorates, nitrates, or permanganates. It is paramagnetic due to the presence of two unpaired electrons in antibonding $\pi^*$ orbitals (Molecular Orbital Theory).

Ozone ($O_3$)

An allotrope of oxygen. It is thermodynamically unstable with respect to oxygen ($\Delta H$ is negative, $\Delta S$ is positive).

$$ 3O_2 \xrightarrow{\text{Silent Electric Discharge}} 2O_3 \quad (\Delta H^\theta = +142 \ kJ/mol) $$

Oxidizing Action: Acts as a strong oxidizing agent liberating nascent oxygen.
Example: Tailing of Mercury ($2Hg + O_3 \rightarrow Hg_2O + O_2$).

Sulphuric Acid ($H_2SO_4$) - The Contact Process

Manufactured by the Contact Process in three steps:

Burning of Sulphur to $SO_2$.
Catalytic Oxidation of $SO_2$ to $SO_3$: This is the key step. $$ 2SO_2(g) + O_2(g) \xrightarrow{V_2O_5, \ 720K, \ 2 \ bar} 2SO_3(g) \quad (\Delta H < 0) $$
Absorption of $SO_3$ in $H_2SO_4$ to give Oleum ($H_2S_2O_7$), which is then diluted to get desired concentration.

4. Allotropes of Sulphur

Rhombic Sulphur ($\alpha$-sulphur): Stable below 369 K. Yellow color.
Monoclinic Sulphur ($\beta$-sulphur): Stable above 369 K.
Transition Temperature: 369 K. Both forms are stable at this temperature.
Structure: Both exist as $S_8$ molecules with a puckered crown shape.
At very high temperatures (~1000 K), $S_2$ is the dominant species and is paramagnetic like $O_2$.

5. Oxoacids of Sulphur

Sulphurous Acid ($H_2SO_3$)

Sulphuric Acid ($H_2SO_4$)

Peroxodisulphuric Acid ($H_2S_2O_8$)
Known as Marshall's Acid. Contains -O-O- linkage.

Pyrosulphuric Acid ($H_2S_2O_7$)
Known as Oleum. Contains -O- linkage.

Group 16 Quiz

Test your concepts on Chalcogens. 10 MCQs with explanations.

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