NEET Physics: Waves – NCERT Masterclass & 100 MCQ Quiz

CHEMICAL BONDING & MOLECULAR STRUCTURE

NCERT Masterclass & NEET Study Module

1. Kössel-Lewis Approach & Octet Rule

Atoms combine to achieve a stable noble gas configuration (8 electrons in the outermost shell). They do this by transferring electrons (Ionic bond) or sharing electrons (Covalent bond).

🔥 NEET Exceptions to Octet Rule:

• Incomplete Octet: Central atom has less than 8 electrons. Examples: $BeCl_2$ (4e⁻), $BF_3$ (6e⁻).
• Expanded Octet: Central atom has more than 8 electrons (due to empty d-orbitals). Examples: $PF_5$ (10e⁻), $SF_6$ (12e⁻), $H_2SO_4$ (12e⁻).
• Odd-Electron Molecules: Molecules with an odd number of total electrons. Examples: $NO$, $NO_2$.

Formal Charge: It helps in selecting the most stable Lewis structure (structure with lowest formal charges is preferred).
$F.C. = (\text{Valence e}^-) - (\text{Non-bonding e}^-) - \frac{1}{2}(\text{Bonding e}^-)$

2. Ionic Bonding & Fajan's Rule

Ionic bonds are formed by electrostatic attraction between cations and anions. Favorable conditions: Low Ionization Enthalpy of metal, High negative Electron Gain Enthalpy of non-metal, and High Lattice Enthalpy.

Fajan's Rule (Covalent character in Ionic Bonds): No ionic bond is 100% pure. Covalent character increases if:

• Size of cation is small (high polarizing power).
• Size of anion is large (high polarizability).
• Charge on cation/anion is high.
• Cation has pseudo noble gas configuration (e.g., $Cu^+, Ag^+$) compared to noble gas configuration (e.g., $Na^+$).

3. VSEPR Theory & Dipole Moment

Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the shape of a molecule based on repulsion between electron pairs in the valence shell. Repulsion order: $lp-lp > lp-bp > bp-bp$.

Dipole Moment ($\mu$): Measures the polarity of a molecule. $\mu = Q \times r$ (Unit: Debye). Symmetrical molecules like $CO_2, BF_3, CCl_4$ have $\mu = 0$ (Non-polar).
Important Exception: $NH_3$ ($1.47 D$) has a higher dipole moment than $NF_3$ ($0.23 D$) because in $NH_3$, the orbital dipole of the lone pair adds to the bond dipoles, while in $NF_3$, they oppose each other.

4. Valence Bond Theory (VBT) & Hybridisation

VBT states that covalent bonds are formed by the overlap of half-filled atomic orbitals.

• Sigma ($\sigma$) Bond: Head-on (axial) overlap. Stronger. Exists independently.
• Pi ($\pi$) Bond: Sideways (lateral) overlap. Weaker. Always exists along with a $\sigma$ bond.

Hybridisation: Mixing of atomic orbitals of slightly different energies to form new equivalent orbitals (hybrid orbitals).

Number of Hybrid Orbitals = No. of $\sigma$ bonds + No. of lone pairs

Examples: $CH_4 (sp^3), C_2H_4 (sp^2), C_2H_2 (sp), PCl_5 (sp^3d), SF_6 (sp^3d^2)$.

5. Molecular Orbital Theory (MOT)

Atomic orbitals combine to form Molecular Orbitals (MOs).
Bonding MOs have lower energy (stable), Antibonding MOs (*) have higher energy.

Bond Order (B.O.) = $\frac{N_b - N_a}{2}$
If B.O. > 0, molecule is stable. If B.O. $\le$ 0, molecule does not exist (e.g., $He_2$).
Magnetic property: If all electrons are paired $\rightarrow$ Diamagnetic. If any unpaired electron is present $\rightarrow$ Paramagnetic.
NEET Pro-Tip: $O_2$ has 16 electrons but is Paramagnetic due to two unpaired electrons in its $\pi^*$ antibonding orbitals.

6. Hydrogen Bonding

Special type of dipole-dipole interaction between a Hydrogen atom (bonded to highly electronegative F, O, or N) and another electronegative atom.

• Intermolecular H-Bond: Between two different molecules (e.g., $H_2O$, $HF$, $NH_3$). Causes unusually high boiling points.
• Intramolecular H-Bond: Within the same molecule (e.g., o-nitrophenol). Generally causes lower boiling points compared to intermolecular.

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🚀 NEET CHEMICAL BONDING MEGA QUIZ (100 MCQ)

Solve the 5 parts below to master VSEPR, Hybridisation, MOT, and Fajans' Rule.

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