HNO3 (nitric acid) has one hydrogen atom, one nitrogen atom, and three oxygen atoms. In the lewis structure of HNO3, there is one double bond and two single bonds around the nitrogen atom, with three oxygen atoms attached to it. The oxygen atom with a double bond has two lone pairs, the left oxygen atom (with which the hydrogen atom is attached) also has two lone pairs, and the right oxygen atom with a single bond has three lone pairs.
Also, there is a negative (-1) charge on the right oxygen atom, and a positive (+1) charge on the nitrogen atom.
Here’s how you can draw the HNO3 lewis structure step by step.
Step #1: draw sketch
Step #2: mark lone pairs
Step #3: mark charges
Step #4: minimize charges
Step #5: minimize charges again (if there are)
Let’s break down each step in detail.
#1 Draw Sketch
- First, determine the total number of valence electrons
Hence, hydrogen has one valence electron, nitrogen has five valence electrons, and oxygen has six valence electrons.
Since HNO3 has one hydrogen atom, one nitrogen atom, and three oxygen atoms, so…
Valence electrons of one hydrogen atom = 1 × 1 = 1
Valence electrons of one nitrogen atom = 5 × 1 = 5
Valence electrons of three oxygen atoms = 6 × 3 = 18
And the total valence electrons = 1 + 5 + 18 = 24
- Second, find the total electron pairs
We have a total of 24 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 = 24 ÷ 2 = 12
- 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 nitrogen and oxygen. Place the least electronegative atom at the center.
Since nitrogen is less electronegative than oxygen, assume that the central atom is nitrogen.
Therefore, place nitrogen in the center and hydrogen and oxygen on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 12 electron pairs. And four bonds are already marked. So we have to only mark the remaining eight electron pairs as lone pairs on the sketch.
Also remember that hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell. And both (nitrogen and oxygen) are the period 2 elements, so they can not keep more than 8 electrons in their last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogen and oxygens. But no need to mark on hydrogen, because hydrogen already has two electrons.
So for top oxygen and right oxygen, there are three lone pairs, and for left oxygen, there are two lone pairs.
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 hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0
For nitrogen atom, formal charge = 5 – 0 – ½ (6) = +2
For top oxygen and right oxygen atom, formal charge = 6 – 6 – ½ (2) = -1
For left oxygen atom, formal charge = 6 – 4 – ½ (4) = 0
Here, both nitrogen and oxygen atoms have charges, so mark them on the sketch as follows:
The above structure is not a stable lewis structure because both nitrogen and oxygen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.
#4 Minimize Charges
Convert a lone pair of the top oxygen atom to make a new N — O bond with the nitrogen atom as follows:
In the above structure, you can see that the central atom (nitrogen) forms an octet. Hence, the octet rule is satisfied.
Now there are still charges on the atoms. But we can not convert a lone pair to a bond because nitrogen can not keep more than 8 electrons in its last shell.
The formal charges on atoms are closer to zero. Also, the above structure is more stable than the previous structures. Therefore, this structure is the most stable lewis structure of HNO3.
Next: SCN– Lewis Structure