CH3– (methanide) has one carbon atom and three hydrogen atoms. In the lewis structure of CH3–, there are three single bonds around the carbon atom, with three hydrogen atoms attached to it, and on the carbon atom, there is one lone pair.
Also, there is a negative (-1) charge on the carbon atom.
Here’s how you can draw the CH3– lewis structure step by step.
Step #1: draw sketch
Step #2: mark lone pairs
Step #3: mark charges (if there are)
Let’s break down each step in detail.
#1 Draw Sketch
- First, determine the total number of valence electrons
Hence, carbon has four valence electrons and hydrogen has one valence electron.
Since CH3– has one carbon atom and three hydrogen atoms, so…
Valence electrons of one carbon atom = 4 × 1 = 4
Valence electrons of three hydrogen atoms = 1 × 3 = 3
Now the CH3– has a negative (-1) charge, so we have to add one more electron.
So the total valence electrons = 4 + 3 + 1 = 8
- Second, find the total electron pairs
We have a total of 8 valence electrons. And when we divide this value by two, we get the value of total electron pairs.
Total electron pairs = total valence electrons ÷ 2
So the total electron pairs = 8 ÷ 2 = 4
- Third, determine the central atom
Here hydrogen can not be the central atom. Because the central atom is bonded with at least two other atoms, and hydrogen has only one electron in its last shell, so it can not make more than one bond.
Hence, here we have to assume that the central atom is carbon.
Therefore, place carbon in the center and hydrogens on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 4 electron pairs. And three C — H bonds are already marked. So we have to only mark the remaining one electron pair as a lone pair on the sketch.
Also remember that carbon is a period 2 element, so it can not keep more than 8 electrons in its last shell. And hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogens. But no need to mark on hydrogen, because each hydrogen has already two electrons.
So for carbon, there is one lone pair.
Mark the lone pair on the sketch as follows:
#3 Mark Charges
Use the following formula to calculate the formal charges on atoms:
Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons
For carbon atom, formal charge = 4 – 2 – ½ (6) = -1
For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0
Here, the carbon atom has a charge, so mark it on the sketch as follows:
In the above structure, you can see that the central atom (carbon) doesn’t form an octet. But in CH3–, carbon has an exception that it does not require eight electrons to form an octet. So no need to worry about the octet rule here.
Now there is still a negative (-1) charge on the carbon atom.
This is okay, because the structure with a negative charge on the most electronegative atom is the best lewis structure. And in this case, the most electronegative element is carbon.
Therefore, this structure is the most stable lewis structure of CH3–.
And since the CH3– has a negative (-1) charge, mention that charge on the lewis structure by drawing brackets as follows:
Next: I2 Lewis Structure