Numerical/Formula
1. TITLE
How to solve Formal Charge Calculations in NEET — Step-by-step
2. PROBLEM TYPE INTRO
This numerical problem belongs to the Chemical Bonding category, specifically focusing on Lewis Structures and stability. To find the formal charge, we use the formula involving valence electrons, lone pair electrons, and bonding electrons. A typical mistake students make is counting electron pairs instead of individual electrons when substituting values into the equation.
3. MCQ
Problem:
Calculate the Formal Charge (F.C.) on the central oxygen atom in an Ozone ($O_3$) molecule.
Given Data:
* Valence electrons of Oxygen ($V$) = $6$
* The Lewis structure of $O_3$ has one double bond and one single bond connected to the central atom.
Options:
A) $0$
B) $+1$
C) $-1$
D) $+2$
4. STEP-BY-STEP SOLUTION
SOLUTION
Given:
* Molecule: Ozone ($O_3$)
* Atom of interest: Central Oxygen atom
* Group number of Oxygen: 16 (Valence electrons, $V = 6$)
To Find:
The Formal Charge (F.C.) on the central oxygen atom.
Formula:
Use this formula to determine the distribution of charge within a molecule, which helps in selecting the most stable Lewis structure (lowest energy).
Step 1: Draw the Lewis Structure and Identify Electrons
First, draw the structure for $O_3$: $\text{O}=\text{O}-\text{O}$.
Analyze the Central Oxygen atom:
* Valence Electrons ($V$): Oxygen has 6 valence electrons in its free state.
* Shared/Bonding Electrons ($S$): The central O forms a double bond (4 electrons) with one O and a single bond (2 electrons) with the other. Total $S = 4 + 2 = 6$ electrons.
* Lone/Non-bonding Electrons ($L$): To complete its octet (8 total surrounding electrons), the central O already has 6 bonding electrons. It needs 2 more. Thus, it has 2 non-bonding electrons (1 lone pair).
Step 2: Substitute Values into the Formula
Step 3: Calculate the Final Value
Answer: $+1$ (Option B)
5. WHERE STUDENTS GO WRONG
- ❌ Mistake 1: Counting electron pairs instead of individual electrons for $L$ and $S$ (e.g., entering "1" for the lone pair instead of "2" electrons).
- ❌ Mistake 2: Calculating the charge for the wrong atom (e.g., the single-bonded terminal oxygen has a charge of $-1$, while the double-bonded terminal oxygen is $0$).
- ❌ Mistake 3: Forgetting to divide the shared electrons ($S$) by 2.
6. QUICK FORMULA REFERENCE
Formulas Used:
- Formal Charge: $F.C. = V - L - \frac{1}{2}S$ — Use to find charge distribution on specific atoms.
- Bond Order (Lewis): $\text{Number of bonds between atoms}$ — Use to estimate stability/enthalpy (Higher order = Higher stability).
7. SIMILAR PROBLEM HINT
"If NEET changes the values, the method stays the same. Practice with: The Nitrite ion ($NO_2^-$). Calculate the formal charge on the central Nitrogen atom." (Hint: The calculation result is 0).
8. NCERT REFERENCE
Based on: NCERT Class 11, Chapter 4 (Chemical Bonding & Molecular Structure), Section 4.1.4, Example of Ozone ($O_3$).
9. CTA
More solved numericals → Chemical Bonding & Molecular Structure Complete Guide
Practice problems → Mock exams