Calculate the formal charges by drawing the Lewis structure for ClNO2.
The Correct Answer and Explanation is:
To calculate the formal charges for ClNO₂, we first draw the Lewis structure. Chlorine nitrite (ClNO₂) consists of one chlorine atom, one nitrogen atom, and two oxygen atoms.
Step 1: Count the total valence electrons
- Cl has 7 valence electrons
- N has 5 valence electrons
- Each O has 6 valence electrons × 2 = 12
Total = 7 + 5 + 12 = 24 valence electrons
Step 2: Choose the central atom
Nitrogen is less electronegative than oxygen, and chlorine usually does not form central bonds unless required. So nitrogen will be the central atom, bonded to both oxygen atoms and the chlorine atom.
Step 3: Sketch the basic skeleton
- Connect Cl to N with a single bond
- Connect N to each O with a single bond initially
This uses 3 bonds × 2 electrons = 6 electrons. We have 18 electrons left to place as lone pairs.
Step 4: Distribute remaining electrons to satisfy octets
- Place lone pairs on both oxygens to give them octets
- Place remaining electrons on chlorine
Now both oxygen atoms have full octets. Nitrogen has 3 bonds (6 electrons), so it needs one more bond to fulfill the octet rule. We can convert one N–O single bond into a double bond.
So the final structure becomes:
- A single bond between Cl and N
- A single bond between N and one O
- A double bond between N and the other O
- Lone pairs on Cl and both O atoms
Step 5: Calculate formal charges using the formula
Formal Charge = Valence electrons − (Lone pair electrons + ½ Bonding electrons)
- Cl: 7 − (6 + 1) = 0
- N: 5 − (0 + 4) = +1
- Single bonded O: 6 − (6 + 1) = −1
- Double bonded O: 6 − (4 + 2) = 0
Final result:
- Cl: 0
- N: +1
- One O: −1
- One O: 0
Explanation
The molecule ClNO₂ has 24 valence electrons. To determine formal charges, we begin by distributing these electrons properly through a Lewis structure. Nitrogen is the logical central atom because it is less electronegative than oxygen and commonly forms multiple bonds. We connect Cl–N and N–O twice. This arrangement initially creates three single bonds which use 6 electrons. We then use the remaining 18 electrons to fill octets, prioritizing oxygen atoms due to their high electronegativity.
After placing electrons, nitrogen is left without a complete octet. To solve this, we convert one of the N–O bonds into a double bond, resulting in a Cl–N single bond, an N=O double bond, and an N–O single bond. This still uses only 24 electrons.
Next, we calculate formal charges. Chlorine has seven valence electrons. In this structure, it shares one bond (2 electrons) and has three lone pairs (6 electrons), so its formal charge is zero. Nitrogen has five valence electrons but shares four bonding electrons and holds no lone pairs. Its formal charge becomes +1. The single-bonded oxygen has six valence electrons, shares one bond (2 electrons), and has three lone pairs (6 electrons), making its formal charge −1. The double-bonded oxygen has four electrons in lone pairs and shares two bonds, so its formal charge is zero.
Thus, the molecule has one positively charged atom (nitrogen) and one negatively charged atom (a single-bonded oxygen), while the others are neutral. This formal charge distribution supports the stability of the molecule and explains its resonance properties. The net charge is zero, consistent with a neutral molecule. Understanding formal charges helps predict chemical reactivity and molecular stability.
