Chemical Bonding and Molecular Structure formulas for JEE Main & Advanced

5 formula entries for Chemical Bonding and Molecular Structure, in the order the chapter builds them. Each shows the formula, when to use it and the first trap to avoid.

Bond Order

Bond Order = \frac{N_{b} - N_{a}}{2}

When to use. Predicting bond strength, bond length, and magnetic properties of diatomic molecules.

Trap. Bond order can be fractional (e.g., H2+ has bond order 0.5).

Formal Charge

Formal Charge = V - L - \frac{B}{2}

where V = valence electrons, L = lone pair electrons, B = bonding electrons

When to use. Evaluating resonance structures and determining the most plausible Lewis structure.

Trap. Formal charge is not the actual charge; it's a bookkeeping tool.

Bond Length and Bond Energy

Bond length decreases and bond energy increases with increasing bond order (qualitative trend, not strict proportionality) for the same pair of atoms.

When to use. Comparing bond strengths and lengths in molecules.

Trap. Bond length decreases with increasing bond order (e.g., C≡C < C=C < C-C).

Dipole Moment

μ = q \times d

When to use. Determining molecular symmetry and polarity.

Trap. Dipole moment is a vector; add bond dipoles vectorially.

Bond Order Calculation from MOT

Bond Order = \frac{N_{b} - N_{a}}{2}

When to use. Predicting bond strength and magnetic behavior of diatomic molecules.

Trap. Remember the MO order for O2: σ1s, σ*1s, σ2s, σ*2s, σ2pz, π2px=π2py, π*2px=π*2py, σ*2pz.

Values to remember

$N_{b}$ and $N_{a}$ from MO diagram Bond Order
$χ_{F} = 4.0$ Electronegativity
$χ_{O} = 3.5$ Electronegativity
$χ_{N} = 3.0$ Electronegativity
$1 D = 3.336 \times 10^{- 30} C \cdot m$ Dipole Moment

Read the full chapter: Chemical Bonding and Molecular Structure for JEE, with what JEE asks and the common traps.