CH3Br (bromomethane) has one carbon atom, three hydrogen atoms, and one bromine atom. In the lewis structure of CH3Br, there are four single bonds around the carbon atom, with three hydrogen atoms and one bromine atom attached to it, and on the bromine atom, there are three lone pairs.
Here’s how you can draw the CH3Br 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, hydrogen has one valence electron, and bromine has seven valence electrons.
Since CH3Br has one carbon atom, three hydrogen atoms, and one bromine atom, so…
Valence electrons of one carbon atom = 4 × 1 = 4
Valence electrons of three hydrogen atoms = 1 × 3 = 3
Valence electrons of one bromine atom = 7 × 1 = 7
And the total valence electrons = 4 + 3 + 7 = 14
- Second, find the total electron pairs
We have a total of 14 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 = 14 ÷ 2 = 7
- 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.
Now we have to choose the central atom from carbon and bromine. Place the least electronegative atom at the center.
Since carbon is less electronegative than bromine, assume that the central atom is carbon.
Therefore, place carbon in the center and hydrogen and bromine on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 7 electron pairs. And four bonds are already marked. So we have to only mark the remaining three electron pairs as lone pairs 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. Hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell. And bromine is a period 4 element, so it can keep more than 8 electrons in its last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogens and bromine. But no need to mark on hydrogen, because each hydrogen has already two electrons.
So for bromine, there are three lone pairs, and for carbon, there is zero lone pair because all three electron pairs are over.
Mark the lone pairs 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 – 0 – ½ (8) = 0
For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0
For bromine atom, formal charge = 7 – 6 – ½ (2) = 0
Here, the atoms do not have charges, so no need to mark the charges.
And in the above structure, you can see that the central atom (carbon) forms an octet. Hence, the octet rule is satisfied.
Therefore, this structure is the stable lewis structure of CH3Br.
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