Chemical Bonding and Molecular Structure: Comprehensive NEET Chemistry Formulae

1. Lewis Structures and Octet Rule

1.1 Octet Rule

• Formula: Atoms combine to complete their octet ($8$ electrons in the outer shell).
• Explanation: The octet rule is the principle that atoms tend to form bonds in such a way that each atom has eight electrons in its valence shell, resulting in the electronic configuration of a noble gas.

Common Misconception: Students often mistakenly apply the octet rule to all elements without considering exceptions like hydrogen (which follows the duplet rule) or elements in and beyond the third period that can have expanded octets.

2. Ionic Bonding

2.1 Lattice Enthalpy

• Formula: Lattice Enthalpy ($\Delta H_{lattice}$)
• • $\text{NaCl (s)}\to {\text{Na}}^{+}(\text{g})+{\text{Cl}}^{-}(\text{g})\Delta H_{lattice}=788\text{ kJ/mol}$
• Explanation: Lattice enthalpy is the energy required to completely separate one mole of a solid ionic compound into its gaseous ions. It indicates the strength of the ionic bonds within a solid lattice structure.

NEET Tip: Remember that a higher lattice enthalpy corresponds to a stronger ionic bond, which also affects the melting and boiling points of ionic compounds.

3. Covalent Bonding

3.1 Bond Order

• Formula: Bond Order ($n$)
• • $\text{Bond Order}=\frac{\text{Number of Bonds}}{\text{Number of Bonding Pairs}}$
• Explanation: Bond order is the number of chemical bonds between a pair of atoms. It indicates bond strength, with higher bond order implying stronger bonds.

3.2 Bond Enthalpy

• Formula: Bond Enthalpy ($\Delta H$)
• • ${\text{H}}_2\to 2\text{H}\Delta H=435.8\text{ kJ/mol}$
• Explanation: Bond enthalpy is the energy required to break one mole of a particular bond in a gaseous molecule. Stronger bonds have higher bond enthalpy.

Did You Know?: Nitrogen gas (${\text{N}}_2$) has one of the highest bond enthalpies (946 kJ/mol) due to its triple bond, making it very stable.

4. Molecular Geometry and VSEPR Theory

4.1 VSEPR Theory

• Formula: Shapes of Molecules Based on VSEPR
• • Linear: ${\text{AB}}_2$ with bond angle 180°.
  • Trigonal Planar: ${\text{AB}}_3$ with bond angle 120°.
  • Tetrahedral: ${\text{AB}}_4$ with bond angle 109.5°.
• Explanation: The Valence Shell Electron Pair Repulsion (VSEPR) theory explains the shapes of molecules based on the repulsion between electron pairs around the central atom.

NEET Tip: Focus on understanding the shapes of common molecules, such as ${\text{CH}}_4$ (tetrahedral) and ${\text{H}}_2\text{O}$ (bent), as these are frequently tested.

5. Dipole Moment

5.1 Dipole Moment Calculation

• Formula: Dipole Moment ($\mu$)
• • $\mu =q\times r$
• Explanation: The dipole moment is the product of the charge difference and the distance between the charges. It is a measure of the polarity of a molecule.

Real-life Application: The dipole moment of water (${\text{H}}_2\text{O}$) is 1.85 D, which is why water is a polar molecule and an excellent solvent for ionic compounds.

Practice Problems

Problem 1: Calculate the Bond Order of Oxygen Molecule (${\text{O}}_2$)

Solution: The bond order of ${\text{O}}_2$ is 2, as there are two bonds between the oxygen atoms (one sigma and one pi bond).

Problem 2: Determine the Shape and Bond Angle of Ammonia (${\text{NH}}_3$) using VSEPR Theory.

Solution: The shape of ${\text{NH}}_3$ is trigonal pyramidal with a bond angle of approximately 107°.

This summary should help you quickly revise and apply key chemistry formulae related to chemical bonding and molecular structure for your NEET exam preparation.