C2H2 (Acetylene | Ethyne) Lewis Structure
C2H2 (acetylene or ethyne) contains two carbon atoms and two hydrogen atoms. There is a triple bond between carbon atoms and hydrogen atoms are joint with carbon atoms though sigma bonds. There are no lone pairs on carbon or hydrogen atoms. In this tutorial, we are going to learn how to draw the lewis structure of C2H2 step by step.
You can see the lewis structure of C2H2 in above figure and you can see it is a simple structure. Now, we are going to draw that C2H4 lewis structure step by step.
Steps of drawing the lewis structure of C2H2
There are several steps to draw a lewis structure of a molecule. Those steps are used in detail in this tutorial to draw C2H2 lewis structure. Because C2H2 molecule is a simple molecule, those all steps may not be used. However, you can learn basic examples of drawing lewis structures.
- Find total number of electrons of the valance shells of hydrogen and carbon atoms.
- Determine total electrons pairs as lone pairs and bonds
- Selection of center atom and basic sketch
- Mark lone pairs on atoms
- Mark charges on atoms if there are charges.
- Check the stability and minimize charges on atoms by converting lone pairs to bonds to obtain best lewis structure.
Total number of electrons of the valance shells of C2H2
There are only two elements in C2H2; hydrogen and carbon. Hydrogen is a group IA element and has only one electron in its last shell (valence shell). Carbon is a group IVA element in the periodic table and contains four electrons in their last shell. Now we know how many electrons are includes in valence shells of hydrogen and carbon atomss.
- valence electrons given by hydrogen atoms = 1 * 2 = 2
- valence electrons given by carbon atoms = 4 * 2 = 8
- Total valence electrons = 2 + 8 = 10
Total valence electrons pairs
Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells
Total electron pairs are determined by dividing the number total valence electrons by two. For, C2H2, total pairs of electrons are 5 (=10/2) in their valence shells.
Selection of center atom and sketch of C2H2 molecule
To be the center atom, ability of having greater valance and being a electropositive element are important facts. however, from our experience we know that there is a very low possibility that hydrogen being a center atom in a molecule.
- It is straight forward that carbon's valence (4) is greater than hydrogen (1). From that point, carbon has the priority to be the center atom.
- When comparing electropositive characteristics of hydrogen and carbon atoms, hydrogen is more electropositive.
- From above two cases, case 1 is more important than case 2 in C2H2. Hence, carbon atoms become center atoms and basic sketch can be figured as below.
Mark lone pairs on carbon and hydrogen atoms
After deciding the center atom and basic sketch of C2H2 molecule, we can start to mark lone pairs on atoms. Remember that, there are total of five electron pairs to mark on atoms.
- There are already three sigma bonds in the above drawn sketch. Now only 2 (5-3) electron pairs are remaining to mark on atoms.
- Usually as a theory, those remaining electron pairs should be first marked on outside atoms. In C2H2, hydrogen atoms are the outside atoms. Hydrogen atoms cannot keep more than two electron in its last shell. There are already two electrons in hydrogen atoms in our drawn sketch. Therefore, we cannot mark those electrons pairs on hydrogen atoms.
- Therefore, then mark those two electrons pairs on one carbon atom. Then all remained lone pairs are finished and there are no more lone pairs to mark on other carbon atom.
Mark charges on atoms
In above sketch, one carbon atom will take +2 charge while other carbon atom will take -2 charge. But, there are no charges on hydrogen atoms.
Check the stability and minimize charges on atoms by converting lone pairs to bonds
If there are charges on atoms and if those charges can be reduced by converting lone pairs to bonds, we should do that to obtain the best stable lewis structure. In the above structure, there ar charges on both carbon atoms. Also one carbon atom (with -2 charge) has 2 lone pairs which can be converted to make bonds with other carbon atom to reduce charges.
In the last structure, there are no charges on atoms. Therefore, this structure should be the lewis structure of C2H2 (acetylene / ethyne).
Questions
Does acetylene lewis structure contains a triple bond and lone pairs?
There is a double bond between two carbon atoms in acetylene. There are no lone pairs on carbon and hydrogen atom in their valence shells because all valence electrons are contributed to make bonds.
Can I decide the hybridization of carbon atoms by looking the lewis structure of acetylene.
Yes. Observe how many sigma bonds and lone pairs exist around a carbon atom and add them. There are only two sigma bonds around a carbon atom. Therefore, hybridization of carbon atom is sp2.