Acetylene (C2H2) Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, polar vs non-polar
C2H2 is the chemical formula of the simplest and the first member of the alkyne family. It is most commonly known as acetylene in the scientific world. However, the IUPAC name for acetylene is ethyne. It exists as a colorless gas (molar mass = 26.04 g/mol) at room temperature and has a garlic-like odor.
Acetylene is widely used as a chemical building block, as a fuel, and for welding, purposes owing to its high reactivity and extremely flammable nature.
In this article, you will learn some interesting facts about acetylene (C2H2) such as how to draw its Lewis structure, what is its molecular geometry or shape, electron geometry, bond angle, bond lengths, hybridization, etc.
The Lewis structure of acetylene (C2H2) is made up of two carbon (C) atoms and two hydrogens (H) atoms. An anyone carbon atom can be considered a central carbon. With reference to this central C-atom, there are a total of 2 electron-density regions around it.
Both electron density regions are comprised of bond pairs. Thus, there is no lone pair of electrons on this central C-atom in the C2H2 Lewis dot structure.
If you want to draw the Lewis dot structure of acetylene with us, then what are you waiting for? Immediately grab a piece of paper and a pencil and draw this structure with us following the simple steps given below.
As carbon (C) is present in Group IV A (or 14) of the Periodic Table so it has 4 valence electrons while hydrogen (H) lies at the top of the Periodic Table of elements with a single valence electron only.
∴ C2H2 has two carbon atoms and two atoms of hydrogen. So, the total valence electrons available for drawing the C2H2 Lewis structure = 2(4) +2(1) = 10valence electrons.
2. Choose the central atom
In this second step, usually the least electronegative atom out of all the concerned atoms is chosen as the central atom.
This is because the least electronegative atom is the one that is most likely to share its electrons with the atoms spread around it.
Hydrogen (E.N = 2.20) is less electronegative than carbon (E.N = 2.55) but it cannot be chosen as the central atom because a hydrogen (H) atom can accommodate only 2 valence electrons so it can form a bond with a single adjacent atom only. This denotes that H is always placed as an outer atom in a Lewis structure.
In short, the two C-atoms are placed at the center while the two hydrogens occupy terminal positions in the C2H2 Lewis structure, as shown below.
3. Connect outer atoms with the central atom
Now we need to connect the outer atoms with the central atom of the Lewis structure using single straight lines. So the outer H-atoms are joined to the two C-atoms at the center using straight lines. Also, the two C-atoms are joined to each other, as shown in the diagram below.
Each straight line represents a single covalent bond i.e., a bond pair containing 2 electrons. There are a total of 3 single bonds in the above diagram.
As 3(2) = 6, that means 6 valence electrons are already consumed out of the 10 initially available.
4. Complete the duplet and/or octet of the outer atoms
Considering the left C-atom as the central atom, there is 1 C-atom and 2 H-atoms marked as outer atoms in the Lewis structure of C2H2 drawn so far.
Each H-atom requires a total of 2 valence electrons in order to achieve a stable duplet electronic configuration. A C-H single bond already represents 2 electrons in the vicinity of each hydrogen. Therefore, each H-atom already has a complete duplet. Thus, we do not need to make any changes with regard to the hydrogen atoms in this structure.
In contrast to that, a C-atom needs a total of 8 valence electrons to achieve a stable octet electronic configuration.
A C-C and a C-H bond represent 2(2) = 4 valence electrons. This denotes the outer C atom needs 4 more electrons to complete its octet. Consequently, 4 valence electrons are placed as 2 lone pairs on the outer carbon (C) atom in the C2H2 Lewis structure, as shown below.
5. Complete the octet of the central atom and make a covalent bond if necessary
Total valence electrons used till step 4 = 3 single bonds + electrons placed around the outer C-atom, shown as dots = 3(2) + 4 = 10 valence electrons.
Total valence electrons – electrons used till step 4 = 10 – 10 = 0 valence electrons.
All the valence electrons initially available are already consumed. But the problem is that there are only two single bonds around the central C-atom which mean 2(2) = 4 valence electrons. It is thus short of 4 electrons to achieve a stable octet.
An easy solution to this problem is to convert 2 lone pairs present on the adjacent C-atom into 2 covalent chemical bonds between both the carbon atoms.
In this way, in the structure shown above, in addition to a complete duplet of the outer H-atoms, the two C-atoms at the center also have a complete octet with 1 mutually shared triple covalent bond and a C-H single bond at each terminal.
Also, both the C-atoms are now equivalent with the same kind of bonding on each side, perfectly in line with what we told you at the beginning i.e., anyone C-atom can be considered a central atom in acetylene structure.
The final step is to check the stability of the Lewis structure obtained in this step. Let us do that using the formal charge concept.
6. Check the stability of the C2H2 Lewis 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 charge can be calculated using the formula given below.
What are the electron and molecular geometry of C2H2?
C2H2 has an identical electron and molecular geometry or shape i.e., linear. Considering any one carbon atom as the central atom, it is bonded to a C-H group on one side and an H-atom on the other.
There are a total of 2 electron density regions around the central C-atom and there is no lone pair of electrons on this atom. As a result, the acetylene (C2H2) molecule experiences no distortion in its shape and geometry.
An easy way to find the shape and geometry of the molecule is to use the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on it.
It is used to predict the geometry or shape of a molecule using the VSEPR concept.
AXN notation for C2H2 molecule
A in the AXN formula represents the central atom. In the C2H2 molecule, carbon is present at the center so A = Carbon.
X denotes the atoms bonded to the central atom. In C2H2, a hydrogen (H) is attached on one side while a CH group is attached on the other end. The CH group is considered 1 region of electron density in this molecule. In short X= 1+1 =2 for C2H2.
N stands for the lone pairs present on the central atom. As per the Lewis structure of the C2H2 molecule, there is no lone pair on central C so N=0.
Hence, the AXN generic formula for the C2H2 molecule is AX2.
Now, you may have a look at the VSEPR chart below.
According to this chart, a molecule with an AX2 generic formula has a linear electron and molecular geometry or shape, as we already noted down for the C2H2 molecule.
Hybridization of C2H2
The hybridization present in a molecule can be determined from the steric number of the central atom of the molecule.
The steric number of central C-atom in C2H2 is 2 so it has sp hybridization.
Refer to the figure and table given below.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The C2H2 bond angle
As the acetylene (C2H2) molecule has a symmetrical linear shape thus the bonded atoms form a mutual bond angle of 180° in this molecule.
A C-C triple bond length is 120.3 pm while the C-H single bond length is 106.0 pm in C2H2.
Acetylene (C2H2) is a non-polar molecule. Zero electronegativity difference exists between two identical C-atoms at the center of the molecule.
However, there is a small electronegativity difference of 0.35 units between the bonded C (E.N = 2.55) and H (E.N = 2.20) atoms in C2H2. The central carbon atoms only slightly attract the shared C-H electron clouds from each side of the molecule.
However, it is due to the symmetrical linear shape of C2H2 that the small C-H dipole moments get canceled equally on opposite ends of the molecule (as shown below). The electron cloud stays uniformly distributed in the molecule overall. Thus acetylene is non-polar with net µ =0.
There are 2 single covalent bonds and 1 triple covalent bond in the Lewis structure of C2H2. Both the single C-H bonds are sigma (σ) bonds while the C≡C triple bond is made up of 1 sigma (σ) bond and 2 pi (π) bonds.
In short, there are a total of 3 sigma bonds and 2 pi bonds in the C2H2 Lewis structure.
What is C2H2 molecular geometry?
The molecular geometry or shape of C2H2 is identical to its electron pair geometry i.e., linear. There are a total of 2 electron density regions around the central C-atom in C2H2 while no lone pair is present on this central atom.
The AXN generic formula for this molecule is AX2. No lone pair-lone pair and lone pair-bond pair electronic repulsions exist in the molecule.
Thus the molecule (C2H2) occupies a symmetrical linear shape where all the bonded atoms form a mutual bond angle of 180°.
Does the C2H2 molecule have isomers?
No there are no isomers of the C2H2 molecule. There is only one atomic arrangement possible for the bonded atoms in the C2H2 molecule.
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