Reactivity with Metals
Master Oxides, Sulfides, Fajans' Rules, and the generation of H₂S.
Group 16 elements (Chalcogens) are highly electronegative. When they react with electropositive metals, they act as powerful electron acceptors. Because they possess six valence electrons ($ns^2 np^4$), they require exactly two more electrons to complete their stable noble gas octet, leading to the formation of binary compounds where the chalcogen is in a -2 oxidation state.
1. The Strict -2 Oxidation State
Whether it is Oxygen forming oxides, Sulfur forming sulfides, or Tellurium forming tellurides, all elements of Group 16 exhibit an oxidation state of -2 when bonded to active metals.
(Sodium sulfide: Na is +1, S is -2)
Exceptions: Oxygen is so electronegative that it can sometimes force active metals to form Peroxides (where O is -1, e.g., $Na_2O_2$) and Superoxides (where O is -1/2, e.g., $KO_2$). However, the standard, most stable state is the oxide ($O^{2-}$).
2. The Nature of the Bond: Fajans' Rules
Not all metal chalcogenides are purely ionic. The nature of the bond changes drastically depending on which metal the chalcogen reacts with, and how large the chalcogen atom is. This is governed by Fajans' Rules.
When chalcogens react with highly electropositive Alkali (Group 1) and Alkaline Earth (Group 2) metals, the resulting oxides and sulfides are predominantly ionic.
- They form giant ionic lattices.
- They have extremely high melting points (e.g., $MgO$ melts at $\sim 2852^\circ\text{C}$).
- They are usually soluble in water.
When chalcogens (especially Sulfur, Selenium, and Tellurium) react with transition metals or heavy p-block metals (like $Ag, Cu, Pb, Hg$), the bonds exhibit significant covalent character.
- Because $S^{2-}, Se^{2-}, Te^{2-}$ are very large anions, their electron clouds are easily distorted (polarized) by the metal cations.
- This high polarization leads to covalency.
- Result: These heavy metal sulfides are distinctly insoluble in water and are often vividly colored (mostly black).
Figure 1: Fajans' Rules. Because the $S^{2-}$ ion is much larger than $O^{2-}$, its valence electrons are loosely held and easily pulled (polarized) by the metal cation, leading to covalent orbital overlap.
This covalent character perfectly explains why elements on the left of the periodic table (s-block) are found in nature as oxides/carbonates, while heavy transition metals (like Copper, Zinc, Lead, and Silver) are almost exclusively found in nature as Sulfide Ores (e.g., Copper Pyrites, Zinc Blende, Galena, Argentite).
3. Hydrolysis & The Generation of $H_2S$
Because the Group 16 elements are the more electronegative part of these binary compounds, treating these metal compounds with water or dilute acids results in the generation of the corresponding hydrides ($H_2O, H_2S, H_2Se, H_2Te$).
A. Hydrolysis of Oxides
Active metal oxides react with water to form highly basic metal hydroxides.
B. Hydrolysis of Sulfides
Soluble metal sulfides (like $Na_2S$ or $CaS$) hydrolyze in water to give alkaline solutions and release Hydrogen Sulfide ($H_2S$) gas, which is infamous for its foul, rotten egg smell.
C. Lab Application: Kipp's Apparatus
While heavy metal sulfides are insoluble in pure water, they can be forced to react using strong, dilute acids. In qualitative salt analysis laboratories, $H_2S$ gas is continuously generated using Kipp's Apparatus by reacting Iron(II) sulfide with dilute sulfuric acid.
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