Is H2S Ionic or Covalent? H2S is a covalent compound because, according to Fajan’s Rules, the large size and greater charge on the sulfide anion (S2-) favor the formation of a covalent bond. The small electronegativity difference between the sulfur and hydrogen atoms also contributes to the formation of a covalent bond. H2S consists of two elements, Hydrogen, and Sulfur. Hence, it is a binary compound. Industrially, pure H2S is obtained by separating it from other constituent gases of natural gas. At first glance, H2S may seem to be a very simple molecule. But, this humble compound is known to show superconductivity, i.e., zero resistance to electric current at very low temperatures (< 150 Kelvin) and very high pressures (> 100 GPa). In this article, we carry out a detailed chemical analysis to determine if H2S is Ionic or covalent.
How is a Covalent bond different from an Ionic bond?
A Covalent bond is formed when two atoms share electrons to achieve a stable electronic configuration. In the case of Ionic bonding, the two atoms in question gain and lose electrons to form a negatively charged anion and a positively charged cation, respectively. The cation and the anion gain stability for having a complete octet, and the attraction between the cation and the anion further increases the molecule’s stability. The difference between the two can be further explained by comparing the properties of their compounds.
Conditions for formation of Covalent Bond
- Both atoms should have high ionization potential as ionization potential is proportional to the energy required for ion formation.
- Both atoms should have high electron affinity to assist in sharing electrons.
- The electronegativity difference should be as small as possible because a higher contrast in electronegativity implies more ionic character and less sharing of electrons.
Polarization and Fajan’s Rules
In reality, no chemical bond is 100% ionic, nor is it 100% covalent. The percentage covalent character in a chemical bond is governed by the degree of polarization. Polarization is defined as the tendency of a cation to distort the electron cloud of an anion and vice versa. The covalent character in a chemical bond is proportional to the degree of polarization. Smaller cations induce a high degree of polarization with a large positive charge and larger anions with a large negative charge.
Why is H2S Covalent?
Let us look at the electron configuration of H and S and determine the type of bonding. Electron Configuration of H (Atomic Number 1): 1s1 Electron Configuration of S (Atomic Number 16): [Ne] 3s2 3p4 H is one electron short of achieving the inert electron configuration of Helium (Atomic Number 2), and S is two electrons short of achieving the inert electron configuration of Argon (Atomic Number 18).
On Pauling’s Scale of electronegativity, Hydrogen and Sulfur are assigned values of 2.20 and 2.58, respectively. A higher value indicated a more electronegative atom, and a lower value indicates a more electropositive atom. Let us consider the formation of two H+ cations and one sulfide anion S2-. Both ions are stable as far as electron configuration is concerned. However, notice that the H+ cation is very small (a free proton) while the S2- is a large (170 pm) anion. According to Fajan’s Rules, covalent bonds must be formed between the three atoms. A Lewis structure of H2S must be drawn to visualize the sharing of electrons via covalent bonding.
Bonding in H2S using Lewis structure and VSEPR Theory
We employ the following procedure to determine the structure of H2S: Step 1: Determine the central atom. In our case, we choose S as the central atom. Step 2: Count the total number of valence electrons in the molecule. n1 = 6 (from S) + 2 x 1 (from H) = 8 Step 3: Count the number of electrons needed to fulfill the octet of all atoms. n2 = 8 x (number of non-H atoms) + 2 x (number of H atoms) = 8 x 1 + 2 x 2 = 12 Step 4: Number of bond pairs = (n2 – n1)/2 = 4/2 = 2 Step 5: Count the number of non-bonding electrons. n3 = n1 – (n2 – n1) = 8 – (12 – 8) = 4 Step 6: Number of lone pairs = n3/2 = 4/2 = 2 Now, we are ready to draw the Lewis structure of H2S. First, we place S at the center and place two bond pairs connecting it to the H atoms. The two bond pairs form the covalent bonds connection S to H. Note that the H atoms have achieved the electronic configuration of Ne, but the S atom is still four electrons short. To remedy this, we place the two lone pairs on the S atom. Now, the S atom has achieved the stable, inert electron configuration of Ar.
Step 7: Calculate the Formal Charge on all atoms. We can check the validity of our Lewis structure by using the concept of formal charge. Formal charge on an atom is defined as follows: Formal Charge = Valence Electrons – (0.5 x Bonding Electrons) – Non-Bonding Electrons The number of bonding and non-bonding electrons can be found from the Lewis structure. Formal Charge the S atom = 6 – (0.5 x 4) – 4 = 6 – 6 = 0 Formal Charge for each H atom = 1 – (0.5 x 2) – 0 = 1 – 1 = 0 Total Charge on the molecule = Sum of formal charges on atoms = 0 + 0 + 0 = 0 This is consistent as H2S is indeed an uncharged neutral molecule. Thus, our Lewis structure is correct. After knowing the electron distribution in the molecule, VSEPR theory can be used to determine the molecule’s geometry. The repulsion between the lone pairs on the S atom and the bond pairs of the H atoms will force the molecule to adopt bent geometry.
Properties of H2S
Chemical Properties
- The gas is toxic by inhalation.
- In a polar aprotic solvent, H2S is a better nucleophile than H2O because of S being less electronegative than O.
- It slowly decomposes in the air to form elemental Sulfur.
- H2S is slightly soluble in water because of hydrogen bonding and weak acidity.
- It reacts with metals ions to form metal sulfides.
Physical Properties
- H2S is a colorless gas with a strong odor of rotten eggs.
- A very low melting point (-82° Celsius or -116° Fahrenheit) and a very low boiling point (-60° Celsius or -76° Fahrenheit) indicate the tendency of H2S to remain in the gaseous phase.
- It is a flammable gas.
- It is slightly denser than air.
- H2S can be liquified by confining it under its own vapor pressure.
Uses of H2S
- H2S is used in the qualitative analysis of inorganic compounds. Its presence often indicates the presence of sulfide anions. Bubbling the gas through the compound solution can result in the precipitation of heavy metal ions such as Pb(II), Cu(II), and Hg(II).
- In metallurgy, H2S is bubbled through to precipitate the valuable metal as metal sulfides.
- The nucleophilicity of H2S is exploited to synthesize organosulfur compounds like methanethiol, ethanethiol, and thioglycolic acid.
- H2S is used to manufacture sulfur alkali metal compounds, which in turn are used extensively in the paper industry.
Conclusion
H2S is a covalent compound as electrons are shared between the S and H atoms. The sharing of electrons results in an inert electron configuration for all the atoms. Two bond pairs of electrons are responsible for the formation of 2 single bonds between the S and H atoms. The rest of the valence electrons manifest themselves as non-bonding electrons or lone pairs on the S atom.
