Group 16 Elements: The Chalcogens
Electronic Configuration, Mineral Occurrence, and the Oxygen family.
Moving one step to the right from the Nitrogen family, we encounter Group 16. This group is dominated by Oxygen, the most abundant element in the Earth's crust, making it an essential chapter not just in chemistry, but for understanding the composition of our planet.
1. Meet the Chalcogens
Group 16 of the periodic table includes Oxygen ($O$), Sulfur ($S$), Selenium ($Se$), Tellurium ($Te$), Polonium ($Po$), and the synthetically radioactive Livermorium ($Lv$).
Collectively, the first four elements of this group are known as the Chalcogens (pronounced kal-ko-jens).
The name is derived from the Greek words for brass/copper (chalcos) and born/produced (genes). It essentially means "ore-forming". This is because a vast majority of metal ores found in the Earth's crust are either oxides or sulfides (and occasionally selenides or tellurides).
Figure 1: Group 16 transitions from non-metals (O, S) to metalloids (Se, Te) and finally to radioactive metals (Po, Lv).
2. Electronic Configuration & Anomalous Behavior
The general valence shell electronic configuration of Group 16 elements is $ns^2 np^4$. They have six electrons in their outermost shell, meaning they are just two electrons short of achieving a stable noble gas configuration.
You would expect Ionization Enthalpy to increase as you move right across a period (from Group 15 to Group 16) due to increasing nuclear charge. However, Group 16 elements have lower first ionization enthalpies than their preceding Group 15 counterparts.
Reason: Group 15 elements have extra stable half-filled p-orbitals ($np^3$). In Group 16 ($np^4$), two electrons occupy the same p-orbital. This causes electron-electron repulsion, making it easier to lose one electron to achieve the more stable half-filled $np^3$ state.
3. Occurrence in Nature
The abundance of these elements varies drastically, with Oxygen dominating the entire Earth's crust.
A. Oxygen ($O$)
The most abundant of all the elements on Earth.
- Earth's Crust: Makes up a staggering $46.6\%$ by mass of the Earth's crust (mostly as silica and metal oxides).
- Atmosphere: Dry air contains $20.946\%$ oxygen by volume as diatomic $O_2$ gas.
B. Sulfur ($S$)
Much less abundant ($0.03\% - 0.1\%$ in the crust), but highly concentrated in specific ores.
- Sulfates:
• Gypsum: $CaSO_4 \cdot 2H_2O$
• Epsom salt: $MgSO_4 \cdot 7H_2O$
• Baryte: $BaSO_4$ - Sulfides:
• Galena: $PbS$
• Zinc blende: $ZnS$
• Copper pyrites: $CuFeS_2$
C. Selenium, Tellurium, Polonium & Livermorium
Selenium ($Se$) and Tellurium ($Te$) are rare and are usually found as metal selenides and tellurides accompanying sulfide ores.
Polonium ($Po$) is a highly radioactive element. It occurs in nature only as a decay product of Thorium and Uranium minerals. Livermorium ($Lv$) is a synthetic, man-made radioactive element created in particle accelerators.
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