Chemical bonding: Chemistry Notes for GPAT

Chemical Bonding – GPAT Study Notes

(Chemistry – General Pharmacy Aptitude Test)

1. Simple Explanation

Chemical bonding is the attractive interaction that holds atoms together in a molecule or solid.
Three main types:

Bond type	Key feature	Typical example
Ionic	Electron transfer → charged ions	NaCl
Covalent	Electron sharing	H₂, CH₄
Metallic	Delocalised “sea” of electrons	Cu, Fe

Electronegativity (χ) determines how strongly an atom attracts shared electrons.
Bond polarity arises when the two atoms have different χ.
Hybridisation explains the shape of covalent bonds (sp, sp², sp³, etc.).
Molecular Orbital (MO) theory provides a more accurate picture of bonding, bond order, and magnetism.

2. Key Points

Topic	Key Points
Ionic Bonding	• Formed between metal (low χ) and non‑metal (high χ). • Electrons transfer → cation + anion. • Strong electrostatic attraction → high melting/boiling points. • Dissolves in polar solvents → conducts electricity.
Covalent Bonding	• Shared electrons between non‑metals. • Single, double, triple bonds – 2, 4, 6 electrons shared. • Polarity depends on Δχ: • Δχ < 0.5 → non‑polar • 0.5 < Δχ < 1.7 → polar covalent • Δχ > 1.7 → ionic‑like (often considered ionic). • Lewis structures: dots for valence electrons.
Hybridisation	• Mixing of atomic orbitals to form new hybrid orbitals. • sp → 2 linear → 180°. • sp² → 3 trigonal planar → 120°. • sp³ → 4 tetrahedral → 109.5°. • Determines bond angles and molecular geometry.
VSEPR Theory	• Electron‑pair geometry predicted from the number of electron domains around the central atom. • Shapes: Linear, Trigonal Planar, Tetrahedral, Trigonal Bipyramidal, Octahedral. • Bond angles modified by lone pairs (e.g., H₂O → 104.5°).
Metallic Bonding	• Delocalised electrons in a lattice of positive ions. • Good electrical and thermal conductivity. • Malleable & ductile.
Coordinate (Dative) Covalent Bond	• Both electrons donated by the same atom. • Forms in complexes (e.g., NH₃ → NH₄⁺).
Hydrogen Bond	• H attached to highly electronegative atom (F, O, N) interacting with lone pair on another electronegative atom. • Strength: 5–30 kJ mol⁻¹. • Important for water properties, DNA base pairing.
Bond Energy & Enthalpy	• Bond dissociation energy (BDE) = energy to break a bond in gas phase. • Bond enthalpy of formation → ΔH_f° of compounds.
Molecular Orbital (MO) Theory	• Bonds formed by constructive interference → bonding MO. • Antibonding MO from destructive interference. • Bond order = (n_bonding – n_antibonding)/2. • Predicts magnetic properties (paramagnetic vs diamagnetic).

3. Worked Example

Problem

Predict the type of bond, bond polarity, and molecular geometry for nitrous oxide (N₂O).
Also, draw its Lewis structure and determine the hybridisation of the central atom.

Step‑by‑Step Solution

Step	Action	Result
1. Identify atoms & valence electrons	N (5 e⁻), O (6 e⁻) → total 5+5+6 = 16 e⁻ (including the lone pair on terminal N).	16 valence electrons.
2. Arrange atoms	Linear arrangement suggested by formula N–N–O.
3. Draw single bonds	N–N (1 e⁻ each side) + N–O (1 e⁻). Count 6 e⁻ used.
4. Complete octets	Remaining 10 e⁻ → 5 lone pairs. Place 2 on O, 3 on terminal N, 0 on central N.
5. Check formal charges	Terminal N: 5 valence – (3 lone + 1 bond × 1/2) = 0. Central N: 5 – (0 lone + 2 bonds × 1/2) = +1. O: 6 – (4 lone + 1 bond × 1/2) = –1.
6. Adjust for minimum F.C.	Replace one N–O single bond with a double bond → N–O double, N–N single. Now F.C. of central N becomes 0.
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