1. What is Atomic Structure ?
Atomic structure refers to the constitution of an atom, which is the smallest unit of ordinary matter that forms a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.
Basic Concept
An atom consists of three primary subatomic particles, each defined by its mass and electrical charge:
- Protons: Found in the central nucleus, protons have a positive charge (+1). The number of protons determines the atomic number (Z) and identifies the element.
- Neutrons: Also located in the nucleus, these have no charge (neutral). They add mass and act as a “buffer” to stabilize the protons.
- Electrons: These are much smaller particles with a negative charge (-1). They reside in the space surrounding the nucleus.
Table of Contents
2. Atomic Models
- Bohr’s Model
- Electrons revolve in fixed orbits (energy levels or shells: K, L, M, N)
- Energy is quantized
- Quantum Mechanical Model
- Electrons exist in orbitals (probability clouds)
- Defined by quantum numbers:
- Principal (n)
- Azimuthal (l)
- Magnetic (m)
- Spin (s)
Electronic Configuration
- Arrangement of electrons in shells and subshells
- Governed by:
- Aufbau Principle (lowest energy first)
- Pauli Exclusion Principle (max 2 electrons/orbital)
- Hund’s Rule (maximize unpaired electrons)
Example:
- Sodium (Na): 1s² 2s² 2p⁶ 3s¹
Periodic Properties (Important for Engineering Materials)
- Atomic Radius → size of atom
- Ionization Energy → energy to remove electron
- Electron Affinity → tendency to gain electron
- Electronegativity → attraction for electrons
These properties influence bonding and material behavior.
2. Interatomic Bonding
Atoms combine to form stable structures through bonding.
🔸 Primary Bonds (Strong Bonds)
1. Ionic Bond
- Formed by transfer of electrons
- Between metal and non-metal
- Example: NaCl
Properties:
- High melting point
- Brittle
- Good electrical conductivity in molten state
2. Covalent Bond
- Formed by sharing of electrons
- Between non-metals
Properties:
- Strong and directional
- Low electrical conductivity
- Example: Diamond, Silicon
3. Metallic Bond
- “Sea of free electrons” around positive ions
Properties:
- High electrical & thermal conductivity
- Ductile and malleable
- Example: Iron, Copper
🔸 Secondary Bonds (Weak Bonds)
1. Van der Waals Forces
- Weak attraction between molecules
- Important in polymers
2. Hydrogen Bonding
- Stronger than Van der Waals
- Occurs in water, polymers
3. Energy vs Interatomic Distance Curve
📌 Key Concepts:
- Attractive Forces → pull atoms together
- Repulsive Forces → push atoms apart
- Equilibrium दूरी (r₀) → stable spacing
- Bond Energy → energy required to separate atoms
👉 This curve explains:
- Elasticity
- Thermal expansion
- Strength of materials
4. Relation Between Bonding and Material Properties
| Bond Type | Strength | Electrical Conductivity | Example Materials |
|---|---|---|---|
| Ionic | High | Low (solid) | Ceramics |
| Covalent | Very High | Very Low | Diamond, Si |
| Metallic | Moderate | High | Metals |
| Secondary | Low | Very Low | Polymers |
🔹 5. Engineering Significance
- Mechanical Properties:
- Strong bonds → high strength & hardness
- Electrical Properties:
- Free electrons → conductivity (metals)
- Thermal Properties:
- Bond strength affects melting point
- Material Selection:
- Engineers choose materials based on bonding type