Introduction
Nuclear chemistry deals with the study of the composition of atomic nucleus and nuclear transformations. It includes processes like radioactivity, artificial transmutations, nuclear fission and nuclear fusion. The energies involved in nuclear processes are millions of times greater than those in ordinary chemical reactions.
Radioactivity
Radioactivity is a process in which nuclei of certain elements undergo spontaneous disintegration without excitation by any external means. Elements whose atoms disintegrate and emit radiations are called radioactive elements.
Historical Background
Henry Becquerel (1891)
Discovered radioactivity from potassium uranyl sulphate
Observed spontaneous emission of invisible, penetrating rays
Marie & Pierre Curie
Named the phenomenon "Radioactivity"
Discovered Radium from pitchblende (3 million times more radioactive than uranium)
Types of Radioactivity
Natural Radioactivity
Substance emits radiations by itself
Artificial Radioactivity
Substance starts emitting radiations on exposure to rays from natural radioactive substances
Types of Radioactive Rays
When radioactive emissions are subjected to electric or magnetic fields, they split into three types: α, β and γ-rays.
| Characteristic | α-Ray | β-Ray | γ-Ray |
|---|---|---|---|
| Charge & Mass | +2 charge, 4 unit mass | -1 charge, negligible mass | No charge, negligible mass |
| Identity | Helium nuclei (₂He⁴) | Electron (₋₁e⁰) | High energy photons |
| Magnetic Field | Deflected towards cathode | Deflected towards anode | Not deflected |
| Velocity | 1/10 of light | Same as light | Same as light |
| Ionizing Power | Very high (100× β-rays) | Low (100× γ-rays) | Very low |
| Penetrating Power | Low | 100× α-particles | 10× β-particles |
Theory of Radioactive Disintegration
Rutherford and Soddy (1903) postulated that radioactivity is a nuclear phenomenon and all radioactive changes occur in the atomic nucleus.
Main Points
Atomic nuclei of radioactive elements are unstable
Disintegration is spontaneous and constant
Rate unaffected by temperature, pressure, chemical combination
During disintegration, atoms of new elements (daughter elements) are formed
Types of Disintegration
α-particle Emission
Mass decreases by 4 units
Atomic number decreases by 2 units
ᴡZᴬ → ᴡ⁻⁴Z⁻²ᴬ + ₂He⁴
β-particle Emission
Mass remains same
Atomic number increases by 1 unit
Due to neutron decay: n → p + e⁻
ᴡZᴬ → ᴡZ⁺¹ᴬ + ₋₁e⁰
γ-ray Emission
No change in mass or atomic number
Excess energy released as electromagnetic radiation
Does not produce new element
Group Displacement Law
Soddy, Fajans and Russell (1911-1913) observed patterns in element displacement during radioactive decay.
α-particle Emission
New element displaced two positions left in periodic table
Atomic number decreases by 2
β-particle Emission
New element displaced one position right in periodic table
Atomic number increases by 1
Positron Emission
New element displaced one position left in periodic table
Radioactive Disintegration Series
Natural radioactivity continues until stable nuclei are formed. All nuclei from initial to final stable element constitute a disintegration series.
| Series | Formula | Parent | Half-life (yrs) | End Product | Lost Particles |
|---|---|---|---|---|---|
| Thorium (Natural) | 4n | ²³²₉₀Th | 1.39×10¹⁰ | ²⁰⁸₈₂Pb | 6α, 4β |
| Neptunium (Artificial) | 4n+1 | ²⁴¹₉₄Pu | 2.2×10⁶ | ²⁰⁹₈₃Bi | 8α, 5β |
| Uranium (Natural) | 4n+2 | ²³⁸₉₂U | 4.5×10⁹ | ²⁰⁶₈₂Pb | 8α, 6β |
| Actinium (Natural) | 4n+3 | ²³⁵₉₂U | 7.07×10⁸ | ²⁰⁷₈₂Pb | 7α, 4β |
Nuclear Structure and Forces
Nuclear Composition
Nucleus composed of protons and neutrons (nucleons)
Nuclear forces hold nucleons together
10²¹ times stronger than electrostatic forces
Nuclear Forces
Short-range forces (operate within ~10⁻¹⁵ m)
Drop rapidly to zero at ~10⁻¹³ cm
Not governed by inverse square law
Yukawa's Theory
Nucleons held together by rapid exchange of π-mesons
π-mesons: mass = 275×electron, charge = +1, 0, -1
These forces called exchange forces
Nuclear Stability
Nuclides can be grouped as stable or unstable based on nuclear stability factors.
Factors Affecting Stability
Binding Energy
Mass defect = Total mass of nucleons - Observed atomic mass
Binding energy = Mass defect × 931 MeV
Maximum for iron (8.7 MeV/nucleon)
Packing Fraction
= (Actual isotopic mass - Mass number)/Mass number × 10⁴
Negative value indicates stability
Least for Fe (negative), highest for H (~78)
Magic Numbers
Protons: 2, 8, 20, 28, 50, 82, 114
Neutrons: 2, 8, 20, 28, 50, 126, 184, 196
Nuclei with magic numbers are particularly stable
Neutron-Proton Ratio
Close to 1 for low Z elements
>1 for higher Z elements
Nuclei outside stability belt are unstable
Rate of Radioactive Decay
"The quantity of a radioelement which disappears in unit time (rate of disintegration) is directly proportional to the amount present."
-dN/dt = λN
Nₜ = N₀e^(-λt)
log(N₀/Nₜ) = λt/2.303
λ = 2.303/t × log(N₀/Nₜ)
Important Time Periods
Half-life (t₁/₂)
Time for half of radioactive material to decay
t₁/₂ = 0.693/λ
Independent of amount of radioelement
Average Life (T)
Sum of lives of nuclei / Total number of nuclei
T = 1/λ = t₁/₂/0.693 = 1.44 × t₁/₂
Activity and Units
Specific Activity
Rate of decay per gram of radioactive isotope
= λN/m
Units of Radioactivity
Curie (Ci): 3.7×10¹⁰ dps
Rutherford (rd): 10⁶ dps
Becquerel (Bq): 1 dps
Artificial Transmutation
Conversion of one element into another by artificial means, i.e., by bombarding with fundamental particles.
Projectiles Used
α-particle
₂He⁴
Proton
₁H¹
Deutron
₁H² or ₁D²
Neutron
₀n¹ (best projectile - no charge)
Types of Nuclear Reactions
By α-particles
(α, n) type: ₄Be⁹ + ₂He⁴ → ₆C¹² + ₀n¹
(α, p) type: ₉F¹⁹ + ₂He⁴ → ₁₀Ne²² + ₁H¹
By Protons
(p, n) type: ₁₅P³¹ + ₁H¹ → ₁₆S³¹ + ₀n¹
(p, γ) type: ₆C¹² + ₁H¹ → ₇N¹³ + γ
By Neutrons
(n, p) type: ₁₃Al²⁷ + ₀n¹ → ₁₂Mg²⁷ + ₁H¹
(n, γ) type: ₉₂U²³⁸ + ₀n¹ → ₉₂U²³⁹ + γ
Nuclear Fission and Fusion
Nuclear Fission
Definition
Splitting of heavier atom into smaller fragments
Discovered by Hahn and Strassman
₉₂U²³⁵ + ₀n¹ → ₅₆Ba¹⁴⁰ + ₃₆Kr⁹³ + 3₀n¹
Applications
Atomic bomb
Nuclear reactors
Energy production
Nuclear Fusion
Definition
Lighter nuclei fuse to form heavier nuclei
Requires very high temperatures (millions of degrees)
4₁H¹ → ₂He⁴ + 2₊₁e⁰ + Energy
Applications
Hydrogen bomb
Stellar energy (sun)
Potential future energy source
| Aspect | Nuclear Fission | Nuclear Fusion |
|---|---|---|
| Elements | Heavy elements | Light elements |
| Process | Splitting of heavy nucleus | Fusion of lighter nuclei |
| Temperature | Ordinary temperature | Very high (10⁸ °C) |
| Energy per reaction | 200 MeV per fission | 3-24 MeV per fusion |
| Efficiency | Comparatively less | High (4× fission) |
| Control | Can be controlled | Cannot be controlled |
Applications of Radioactivity
Medical Applications
Cancer treatment (Radiotherapy)
Sterilization of medical equipment
Tracer studies in metabolic processes
Industrial Applications
Thickness measurement of sheets
Detection of flaws in metal castings
Food preservation
Agricultural Applications
Pest control
Genetic improvements in crops
Fertilizer uptake studies
Archaeological Applications
Carbon dating (¹⁴C)
Potassium-argon dating
Uranium-lead dating
Important Points to Remember
Key Points for JEE Main
- Radioactivity: Spontaneous nuclear disintegration
- α-particles: ₂He⁴, +2 charge, low penetration
- β-particles: ₋₁e⁰, -1 charge, medium penetration
- γ-rays: No charge, high penetration
- Displacement law: α→2 left, β→1 right in periodic table
- Half-life: t₁/₂ = 0.693/λ, independent of amount
- Average life: T = 1/λ = 1.44 × t₁/₂
- Fission: Heavy nucleus splits, controlled
- Fusion: Light nuclei fuse, requires high temperature
- Binding energy: Maximum for Fe (8.7 MeV/nucleon)
Do's
Calculate half-life using λ = 0.693/t₁/₂
Apply displacement law to find new elements
Use binding energy formula: Δm × 931 MeV
Remember mass and atomic number changes
Don'ts
Confuse α, β, γ properties
Forget to balance nuclear equations
Mix up fission and fusion processes
Ignore units in radioactivity calculations
JEE Main Weightage
This chapter typically carries 1-2 questions in JEE Main, making it a moderate-weightage chapter. Questions often focus on half-life calculations, nuclear reactions, and types of radioactive decay.
Weightage
Moderate (1-2 questions)