Methanethiol (CH3SH) Lewis structure, molecular geometry or shape, electron geometry, bond angles, hybridization, polar or nonpolar, formal charges
CH3SH is the chemical formula for methanethiol, also known as thiomethanol or methyl mercaptan. CH3SH exists as a volatile, colorless liquid with a distinctive unpleasant odor similar to that of rotten cabbage or garlic.
On account of its strong, pungent odor, methanethiol (CH3SH) is added to odorless gases such as natural gas as a safety measure against gas leaks.
In this article, we will discuss how to draw the Lewis dot structure of methanethiol (CH3SH), what is its molecular geometry or shape, electron geometry, bond angles, hybridization, formal charges, polarity, etc.
So let’s begin! We wish you a helpful learning experience.
The Lewis dot structure of methanethiol (CH3SH) comprises a carbon (C) atom at the center. It is single-covalently bonded to three hydrogens (H) atoms and a thiol (SH) functional group at the sides. There is no lone pair of electrons on the central C-atom, while the S-atom carries 2 lone pairs of electrons in the CH3SH Lewis dot structure.
You can easily draw the Lewis structure of methanethiol (CH3SH) by following the simple steps given below.
Steps for drawing the Lewis dot structure of CH3SH
1. Count the total valence electrons present in CH3SH
The three distinct elements present in CH3SH are carbon, hydrogen and sulfur.
If we look at the Periodic Table of Elements, carbon (C) is located in Group IV A (or 14) of the Periodic Table, which implies that it has a total of 4 valence electrons.
In contrast, sulfur (S) is present in Group VI A (or 16), containing 6 valence electrons in each atom.
However, hydrogen (H) lies at the top of the Periodic Table, containing 1 valence electron only.
Total number of valence electrons in hydrogen = 1
Total number of valence electrons in carbon = 4
Total number of valence electrons in sulfur = 6
The CH3SH molecule comprises 1 C-atom, 4 H-atoms and 1 S-atom.
∴ Therefore, the total valence electrons available for drawing the Lewis dot structure of CH3SH = 1(4) + 4(1) + 1(6) = 14 valence electrons.
2. Find the least electronegative atom and place it at the center
By convention, the least electronegative atom out of all those available is chosen as the central atom while drawing the Lewis structure of a molecule.
The least electronegative atom can easily form covalent bonds with other atoms by sharing its electrons.
Among the atoms present in CH3SH, hydrogen (E.N = 2.20) is less electronegative than both carbon (E.N = 2.55) and sulfur (E.N = 2.58).
However, the H-atom is an exception as it cannot be chosen as the central atom in any Lewis structure. It can accommodate a total of 2 valence electrons, forming a single covalent bond with 1 adjacent atom only.
Therefore, we select the second option, i.e., a C-atom as the central atom in the CH3SH Lewis structure. It is surrounded by 3 H-atoms and an S-atom, as shown below.
You may also note that CH3SH is characterized by its thiol (SH) functional group. Hence, the fourth H-atom is placed next to the sulfur atom and not the central C-atom in the CH3SH Lewis structure.
3. Connect the outer atoms with the central atom
In this step, all the outer atoms are joined to the central C-atom using single straight lines. However, the H-atom next to sulfur is only joined to this S-atom as it can form a single covalent bond only.
A straight line represents a single covalent bond, i.e., a bond pair containing 2 electrons.
In the above structure, there are a total of 5 single bonds, i.e., 5(2) = 10 valence electrons are already consumed out of the 14 initially available.
Now let’s see in the next steps where to place the remaining 4 valence electrons in the CH3SH Lewis dot structure.
4. Complete the octet and/or duplet of the outer atoms
An S-atom needs a total of 8 valence electrons in order to achieve a stable octet electronic configuration.
A C-S and an S-H bond represent 4 valence electrons already present around the terminal S-atom in the CH3SH Lewis dot structure.
Therefore, the remaining 4 valence electrons are placed as 2 lone pairs around the sulfur atom to complete its octet.
In contrast, each H-atom already has a complete duplet, possessing 2 valence electrons, so we do not need to make any changes w.r.t the H-atoms in the above structure.
5. Complete the octet of the central atom
Total valence electrons used till step 4 = 5 single bonds + electrons placed around S-atom, shown as dots = 5(2) + 4 = 14 valence electrons.
Total valence electrons – electrons used till step 4 = 14 – 14 = 0 valence electrons.
As all the valence electrons initially available for drawing the CH3SH Lewis structure are already consumed so there is no lone pair on the central C-atom.
However, we need not worry because this central C-atom already has a complete octet with a total of 4 single bonds, i.e., 8 valence electrons surrounding it.
Hence, the final step is to check the stability of the CH3SH Lewis structure by applying the formal charge concept.
6. Check the stability of Lewis’s structure using the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charges can be calculated using the formula given below.
Formal charge = [valence electrons- nonbonding electrons- ½ (bonding electrons)].
Now let us use this formula and the Lewis structure obtained in step 5 to determine the formal charges present on the CH3SH bonded atoms.
What are the electron and molecular geometry of CH3SH?
The molecular geometry or shape of CH3SH w.r.t the central C-atom is tetrahedral, while w.r.t the S-atom, it is a bent, angular or V-shaped molecule. In contrast, the electronic geometry of CH3SH w.r.t, both the C-atom and the S-atom is tetrahedral.
The presence of 2 lone pairs of electrons on the S-atom leads to strong lone pair-lone pair and lone pair-bond pair electronic repulsions, thus distorting the molecular shape of methanethiol w.r.t the terminal sulfur atom.
Molecular geometry of CH3SH
The molecular geometry or shape of methanethiol (CH3SH) w.r.t the central C-atom is tetrahedral while that w.r.t the S-atom is bent, angular or V-shaped.
To a carbon atom at the center, four different bond pairs are attached like four corners of a tetrahedron, and there is no lone pair of electrons on the respective C-atom.
Contrarily, to the S-atom, 1 C-atom and 1 H-atom are attached as bond pairs, and it also carries 2 lone pairs of electrons. The lone pairs lead to lone pair-lone pair and lone pair-bond pair electronic repulsions.
The bonded atoms tilt away from the center to minimize the electron-repulsive effect. Thus, the molecule occupies a bent or Inverted-V shape w.r.t this sulfur atom, as shown below.
Electron geometry of CH3SH
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule containing a total of 4 electron density regions around the central atom is tetrahedral.
There is no distinction between lone pairs and bond pairs when considering the total electron density regions and, thus, the electronic geometry of a molecule.
Therefore, the electron geometry of CH3SH w.r.t both the C-atom and the S-atom is tetrahedral as all these atoms are surrounded by 4 electron density regions at the sides.
An easy trick to finding a molecule’s electron and molecular geometry is using the AXN method.
AXN is a simple formula representing the number of bonded atoms and lone pairs present on the central atom.
It is used to predict the shape and geometry of a molecule or molecular ion using the VSEPR concept.
As we are considering the less electronegative C-atom as the central atom in the CH3SH Lewis structure so we will explain the AXN formula w.r.t the C-atom only.
AXN notation for CH3SH molecule
A in the AXN formula represents the central atom. In the CH3SH molecule, a carbon (C) atom is present at the center, so A = C.
X denotes the atoms bonded to the central atom. In CH3SH, 3 H-atoms and 1 S-atom are directly bonded to the central C-atom. So X =4 for CH3SH.
N stands for the lone pairs present on the central atom. As per the Lewis structure of CH3SH, the central C-atom has no lone pair of electrons. Thus, N = 0 for CH3SH.
As a result, the AXN generic formula for CH3SH is AX4N0or simply AX4.
Now, you may have a look at the VSEPR chart below.
The VSEPR chart confirms that a molecule with AX4 generic formula possesses an identical electron and molecular geometry or shape, i.e., tetrahedral, as we already noted down for methanethiol (CH3SH).
Hybridization of CH3SH
Both the C-atom and the S-atom are sp3 hybridized in CH3SH.
During chemical bonding in CH3SH, one of the two 2s electrons of carbon shifts to its empty 2p atomic orbital. As a result, the 2s and three 2p atomic orbitals of carbon hybridize to produce four sp3 hybrid orbitals.
Each sp3 hybrid orbital possesses a 25 % s-character and a 75 % p-character. All four sp3 hybrid orbitals are equivalent and contain a single unpaired electron only.
The C-atom uses these sp3 hybrid orbitals to form C-H and C-S sigma bonds with the adjacent atoms.
In contrast, two of the four sp3 hybrid orbitals of sulfur contain paired electrons which are situated as 2 lone pairs on the S-atom in CH3SH. The other two sp3 hybrid orbitals of sulfur-containing a single electron only form C-S and S-H sigma bonds by sp3-sp3 and sp3-s orbital overlap, respectively.
Another shortcut to finding the hybridization present in a molecule is using its steric number against the table given below.
The steric number of both the C-atom and the S-atom in CH3SH is 4, so it has sp3 hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The bond angle of CH3SH
It is due to the tetrahedral shape of CH3SH that each H-C-H bond angle is equal to 109.28° in the methanethiol molecule.
However, the lone pairs present on the S-atom lead to electronic repulsion and thus molecular distortion, which reduces the C-S-H bond angle from an ideal value of 109° to approx. 104.5°.
As per Pauling’s electronegativity scale, a polar covalent bond is formed between two dissimilar atoms having an electronegativity difference between 0.4 to 1.6 units.
The three types of bonds present in CH3SH are C-S, C-H, and S-H.
The C-S bond is almost non-polar as a very small electronegativity difference of 0.03 units is present between the covalently bonded carbon (E.N = 2.55) and sulfur (E.N = 2.58) atoms.
Similarly, a small electronegativity difference of 0.35 units exists between the carbon and hydrogen (E.N = 2.20) atoms in each C-H bond. Thus, the C-H bonds are also only weakly polar, almost non-polar, as per Pauling’s electronegativity scale.
However, the S-H bond is relatively more polar as per an electronegativity difference of 0.38 units between the bonded sulfur and hydrogen atoms.
The more electronegative S-atom thus gains a partial negative charge (δ–) while the adjacent C and H-atoms obtain partial positive charges (δ+), respectively.
Overall, the charged electron cloud stays non-uniformly distributed in the CH3SH molecule.
The unequal C-H, C-S and S-H dipole moments do not get canceled, esp. due to the asymmetric bent shape of methanethiol (CH3SH) w.r.t the S-atom.
Rather, the overall polarity effect adds up to yield a polar CH3SH molecule (net µ > 0).
How do you determine the Lewis Structure of CH3SH?
The Lewis dot structure of methanethiol (CH3SH) displays a total of 14 valence electrons i.e., 14/2 = 7 electron pairs.
Out of the 7 electron pairs, there are 5 bond pairs and 2 lone pairs of electrons.
A C-atom at the center is single covalently bonded to 3 H-atoms and 1 SH group at the sides.
There is no lone pair of electrons on the central C-atom, while both lone pairs are present on the S-atom in the CH3SH Lewis structure.
Which atom in the Lewis Structure of CH3SH has unshared electrons?
The sulfur atom in the Lewis dot structure of CH3SH has 4 unshared electrons, i.e., 4/2 = 2 lone pairs.
What is the molecular geometry or shape of CH3SH?
The molecular geometry or shape of CH3SH w.r.t the central C-atom is tetrahedral, while that w.r.t the S-atom is bent, angular or V-shaped.
What is the electron geometry of CH3SH?
The ideal electron geometry of CH3SH is tetrahedral.
Why is the VSEPR shape of CH3SH different from that of its electronic geometry?
The molecular shape of CH3SH w.r.t the central C-atom is identical to its ideal electron pair geometry, i.e., tetrahedral.
However, the presence of 2 lone pairs of electrons on the S-atom leads to strong lone pair-lone pair and lone pair-bond pair electronic repulsions, which distorts the molecular shape.
As a result, CH3SH occupies a bent, angular, or V-shape w.r.t the sulfur atom, which is different from its electronic geometry.
How is the shape of CH3SH similar to or different from that of CH3OH?
The molecular shape of both CH3SH and CH3OH w.r.t the central C-atom is tetrahedral while that w.r.t the terminal (S or O) atom is bent, angular, or V-shaped.
How is the shape of CH3NH2 similar to or different from that of CH3SH?
The shape of methylamine (CH3NH2) w.r.t the central C-atom is similar to that of CH3SH w.r.t the C-atom, i.e., tetrahedral.
However, the molecular shape of CH3NH2 w.r.t the N-atom is trigonal pyramidal as it has only 1 lone pair of electrons resulting in lone pair-bond pair electronic repulsions.
In comparison, the presence of 2 lone pairs of electrons on the S-atom in CH3SH leads to lone pair-lone pair repulsions in addition to a lone pair-bond pair repulsive effect.
l.p. – l. p repulsions > l.p. – b. p. repulsions.
Thus, the molecule occupies a bent shape w.r.t the S-atom in CH3SH.
The total number of valence electrons available for drawing the methanethiol (CH3SH) Lewis structure is 14.
The molecular geometry or shape of CH3SH w.r.t the C-atom is tetrahedral while that w.r.t the S-atom is bent, angular or V-shaped.
The ideal electronic geometry of CH3SH is tetrahedral.
Both the C-atom and the S-atom are sp3 hybridized in CH3
The H-C-H bond angle is equal to 109.28° while the C-S-H bond angle is equal to 104.5° in CH3SH.
Methanethiol (CH3SH) is a polar molecule (net µ > 0).
Zero or no formal charges on any of the covalently bonded atoms in CH3SH ensure the extraordinary stability of the Lewis structure drawn in this article.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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