Methylium (CH3+) ion Lewis dot structure, molecular geometry
or shape, electron geometry, bond angles, hybridization, polar vs nonpolar
CH3+ represents the chemical formula for the highly reactive and unstable methylium (CH3+) cation. It is prepared in situ in organic synthesis, facilitating the conversion of reactants into products. CH3+ also functions as a reagent for ionization purposes in mass spectrometry.
This article is very important as it provides all the information rarely discussed on the web. You will learn how to draw the Lewis structure of CH3+, what is its molecular geometry or shape, electron geometry, bond angles, hybridization, formal charges and polarity.
So what are you waiting for? Continue reading the article!
The Lewis structure of the CH3+ comprises a C-atom at the center. It is single-covalently bonded to three H-atoms at the sides. The central carbon atom has an incomplete octet in this Lewis structure which makes the methylium cation highly unstable and extremely reactive in nature.
You can draw the best possible Lewis structure of CH3+ by following the simple steps given below.
Steps for drawing the Lewis dot structure of CH3+
1. Count the total valence electrons present in CH3+
The two distinct elements present in CH3+ are carbon and hydrogen.
Carbon (C) is located in Group IVA (or 14) of the Periodic Table of Elements, containing a total of 4 valence electrons.
In contrast, hydrogen (H) lies at the top of the Periodic Table, containing a single valence electron in each atom.
Total number of valence electrons in carbon = 4
Total number of valence electrons in hydrogen = 1
The CH3+ ion comprises 1 C-atom and 3 H-atoms.
An important point to remember is that the CH3+ ion carries a positive one (+1) charge, which means 1 valence electron is removed from this Lewis structure.
∴ Therefore, the total valence electrons available for drawing the Lewis dot structure of CH3+ = 1(4) + 3(1) = 7 -1 = 6 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 or molecular ion.
The least electronegative atom can easily form covalent bonds with other atoms by sharing its electrons.
Hydrogen (E.N = 2.20) is slightly less electronegative as compared to carbon (E.N = 2.55).
However, the H-atom is an exception. It cannot be chosen as the central atom in any Lewis structure because it can only form a single covalent bond with 1 adjacent atom, accommodating a total of 2 valence electrons.
Therefore, we place the C-atom as the central atom in the CH3+ Lewis structure while the three H-atoms are spread around it, as shown below.
3. Connect the outer atoms with the central atom
In this step, the outer atoms, i.e., 3 H-atoms, are joined to the central C-atom using single straight lines.
A straight line represents a single covalent bond, i.e., a bond pair containing 2 electrons.
Thus, all three outer H-atoms already have a complete duplet configuration in the CH3+ Lewis structure drawn till this step.
4. Check if the central atom has a complete octet or not
Total valence electrons used till step 3 = 3 single bonds = 3(2) = 6 valence electrons.
Total valence electrons – electrons used till step 3 = 6 – 6 = 0 valence electrons.
As all the valence electrons initially available for drawing the methylium ion Lewis structure are already consumed so there is no lone pair of electrons on the central C-atom in this structure.
The central C-atom still has an incomplete octet, but as discussed already, the methylium ion is an exception. Its incomplete octet is a sign of its exceptional reactivity and instability.
In conclusion, we can’t do anything about it.
Now as a last step, let’s check the formal charges present on the Lewis structure drawn above.
5. Check the stability of Lewis’s structure using the formal charge concept
The less the formal charge on the atoms of a molecule or molecular ion, 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 4 to determine the formal charges present on the CH3+ bonded atoms.
Non-bonding electrons = no lone pair = 0 electrons
Formal charge = 4-0-6/2 = 4-0-3 = 4-3 = +1
For each hydrogen atom
Valence electrons of hydrogen = 1
Bonding electrons = 1 single bond = 2 electrons
Non-bonding electrons = no lone pairs = 0 electrons
Formal charge = 1-0-2/2 = 1-0-1= 1-1 = 0
As per the above calculation, zero or no formal charges are present on the H-atoms, while the C-atom carries a +1 formal charge which is also the charge present on the methylium cation overall.
This ensures that although unstable, but the structure drawn below is the correct Lewis representation of the methylium (CH3+) cation.
What are the electron and molecular geometry of CH3+?
The molecular geometry or shape of the CH3+ w.r.t the central C-atom is identical to its ideal electronic geometry, i.e., trigonal planar. There is no lone pair of electrons on the central C-atom in CH3+; thus, no distortion is witnessed in its molecular shape or geometry.
Molecular geometry of CH3+
The molecular geometry or shape of the methylium (CH3+) ion is trigonal planar.
The central C-atom is directly bonded to three H-atoms, and there is no lone pair of electrons on the central C-atom. Thus no lone pair-lone pair or lone pair-bond pair electronic repulsions exist in the molecular ion.
Hence no distortion is witnessed in the shape or geometry of the methylium cation.
The bonded atoms form an equilateral triangle around the central C-atom; thus, the name trigonal planar is given to the shape of CH3+, as shown below.
Electron geometry of CH3+
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule or molecular ion containing a total of 3 electron density regions around the central atom is trigonal planar.
In CH3+, the C-atom at the center is surrounded by 3 bond pairs, and it has no lone pair of electrons, making a total of 3 electron density regions. Hence, the ideal electron pair geometry of the methylium cation is also trigonal planar.
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.
AXN notation for CH3+
A in the AXN formula represents the central atom. In CH3+, a carbon (C) atom is present at the center, so A = C.
X denotes the atoms bonded to the central atom. In CH3+, 3 H-atoms are directly bonded to the central C-atom. So X = 3 for CH3+.
N stands for the lone pairs present on the central atom. As per the Lewis structure of CH3+, the central C-atom has no lone pair of electrons. Thus, N = 0 for CH3+.
As a result, the AXN generic formula for CH3+ is AX3N0or simply AX3.
Now, you may have a look at the VSEPR chart below.
The VSEPR chart confirms that a molecule or molecular ion with AX3 generic formula possesses an identical electron and molecular geometry or shape, i.e., trigonal planar, as we already noted down for CH3+.
Hybridization of CH3+
The central C-atom is sp2 hybridized in CH3+.
The electronic configuration of carbon is 1s2 2s2 2p2.
Upon excitation, the carbon loses an electron, so its electronic configuration becomes 1s2 2s2 2p1.
During chemical bonding in CH3+, one of the two 2s electrons of carbon shifts to its empty 2p atomic orbital. As a result, the half-filled 2s and two 2p atomic orbitals of carbon hybridize to produce three sp2 hybrid orbitals.
Each sp2 hybrid orbital possesses a 33.3 % s-character and a 66.7 % p-character. All three sp2 hybrid orbitals are equivalent and contain a single unpaired electron only.
Therefore, carbon uses these sp2 hybrid orbitals to form the C-H sigma bonds by sp2-s orbital overlap, as shown below.
Another shortcut to finding the hybridization present in a molecule or molecular ion is using its steric number against the table given below.
The steric number of the C-atom in CH3+ is 3, so it has sp2 hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The bond angles of CH3+
Each H-C-H bond angle equals the ideal bond angle value of a symmetrical trigonal planar shape, i.e., 120°.
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.
In CH3+, a small electronegativity difference of 0.35 units is present between the covalently bonded carbon (E.N = 2.55) and hydrogen (E.N = 2.20) atoms in each C-H bond.
0.35 units < 0.4 units; therefore, the C-H bonds are considered non-polar, strictly following Pauling’s electronegativity scale.
However, the chemists suggest that no covalent bond is purely non-polar unless it is formed between two identical atoms, such as H-H or O-O.
Carbon being slightly more electronegative than hydrogen, pulls the C-H electron cloud towards itself to a small extent.
There are three C-H bonds in CH3+, so the central C-atom gains a partial negative charge (δ–) while depositing a small partial positive charge (δ+) on each terminal H-atom.
But the twist here is that the small C-H dipole moments get canceled uniformly due to the symmetrical trigonal planar shape of CH3+,as shown below.
This results in an overall non-polar CH3+ having a uniformly dispersed electron cloud surrounding it (net µ= 0).
The Lewis dot structure for methylium (CH3+) ion displays a total of 6 valence electrons, i.e., 6/2 = 3 electron pairs.
All 3 electron pairs are bond pairs, and there is no lone pair of electrons in the CH3+ Lewis structure.
A C-atom at the center is single covalently bonded to 3 H-atoms at the sides. In this way, 3 C-H bonds contain a total of 3(2) = 6 shared electrons, i.e., 3 bond pairs.
What are the charges present on the bonded atoms in the CH3+ Lewis structure?
Zero or no formal charges are present on the three H-atoms, while the central C-atom carries a +1 formal charge in the CH3+ Lewis structure.
How is the Lewis structure of methyl anion (CH3–) different from that of CH3+?
The Lewis dot structure of CH3– contains a total of 8 valence electrons. A C-atom at the center is single covalently bonded to 3 H-atoms, at the sides.
There is 1lone pair of electrons on the central C-atom as opposed to no lone pair present on the central C-atom in the CH3+ Lewis structure. The overall charge present on CH3– ion is -1, while that on CH3+ is +1.
What is the predicted shape, bond angle and hybridization of CH3+?
The methylium (CH3+) cation possesses a trigonal planar shape w.r.t the central C-atom. To a C-atom at the center, three H-atoms are directly attached.
There is no lone pair of electrons on the central carbon; thus, the AXN generic formula for CH3+ is AX3.
The H-C-H bond angles occupy an ideal value of 120°. Correspondingly, the central C-atom is sp2 hybridized in CH3+.
How is the VSEPR shape of CH3+ different from its electron geometry?
The VSEPR shape of CH3+ is identical to its ideal electron geometry as per an AX3 generic formula.
There is no lone pair of electrons on the central C-atom, so no lone pair-lone pair or lone pair-bond pair electronic repulsions exist in the molecular ion, which implies that there is no molecular distortion present in it.
Which species has a pyramidal shape, CH3+, BF3 or CH3–?
The methyl anion (CH3–) possesses a trigonal pyramidal molecular shape. A lone pair of electrons is present on the central C-atom in CH3–, which is directly bonded to three H-atoms.
This leads to strong lone pair-bond pair electronic repulsions, thus distorting the overall molecular shape.
The shape of BF3 is trigonal planar. The central B-atom is bonded to three F-atoms, and there is no lone pair of electrons on the central boron atom.
Similarly, the shape of CH3+ is also trigonal planar.
Which one among CO32-, CH3+, SO32- and NO3– has a different shape than the other three?
Among all the above-mentioned molecular ions, the shape of the sulfite (SO32-) ion is different, i.e., trigonal pyramidal.
In sulfite (SO32-), the central S-atom is directly bonded to three O-atoms, and it has a lone pair of electrons as well which leads to distortion in the overall molecular shape.
In contrast, in carbonate (CO32-), a C-atom at the center is directly bonded to three O-atoms. There is no lone pair of electrons on the central C-atom. The molecular ion thus possesses a trigonal planar shape.
Similarly, in CH3+, a C-atom at the center is directly bonded to three H-atoms in a trigonal planar molecular arrangement.
Likewise, in nitrite (NO3–), an N-atom is directly bonded to 3 O-atoms, forming a trigonal planar molecular shape.
The total number of valence electrons available for drawing the methylium (CH3+) ion Lewis structure is 6.
The molecular geometry or shape of CH3+ w.r.t the central C-atom is identical to its ideal electronic geometry, i.e., trigonal planar.
The central C-atom is sp2 hybridized in CH3+.
CH3+ is a non-polar molecular ion as the equal and opposite small C-H dipole moments get canceled uniformly.
Zero or no formal charges are present on the H-atoms in CH3+, while the central C-atom carries a +1 formal charge which accounts for the overall charge present on the methylium cation.
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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