Chapter 11: Nuclear Chemistry – Study Notes

Chapter 11: Nuclear Chemistry

Introduction to Nuclear Chemistry

Nuclear chemistry is the study of the structure of atomic nuclei and the changes they undergo. Unlike chemical reactions, which involve electrons, nuclear reactions involve changes in the nucleus and can result in the emission of radiation and the transformation of elements.

• Nuclear reactions can change one element into another.

• These reactions often release large amounts of energy.

Nuclear Decay and Nuclear Reactions

Nuclear Decay

Nuclear decay occurs when an unstable atomic nucleus loses energy by emitting radiation. This process can change the mass number and/or atomic number of the nucleus.

• Particles emitted include alpha (α), beta (β), or gamma (γ) radiation.

• Mass number and atomic number may change during decay.

Nuclear Reactions

A nuclear reaction is a process in which two nuclei, or a nucleus and a subatomic particle, collide to produce one or more new nuclei and often other particles. These reactions are described by nuclear equations.

• General form: Radioactive Nucleus → New Nucleus + Radioactive Particle

Balancing Nuclear Equations

In a balanced nuclear equation, the sum of the mass numbers and the sum of the atomic numbers (charges) are equal on both sides of the equation.

• Mass number (A): total number of protons and neutrons.

• Atomic number (Z): number of protons (charge).

Types of Nuclear Radiation

Alpha (α) Emission

Alpha emission occurs when a nucleus emits an alpha particle, which consists of 2 protons and 2 neutrons (identical to a helium-4 nucleus).

• Symbol: $^4_2\text{He}$ or $\alpha$

• Charge: +2

• Low penetration; stopped by paper or skin.

Example:

• $^{232}_{90}\text{Th} \rightarrow ^4_2\text{He} + ^{228}_{88}\text{Ra}$

Beta (β) Emission

Beta emission occurs when a neutron in the nucleus converts to a proton and emits a beta particle (an electron).

• Symbol: $^0_{-1}\text{e}$ or $\beta$

• Charge: -1

• Moderate penetration; stopped by aluminum.

Example:

• $^{131}_{53}\text{I} \rightarrow ^{131}_{54}\text{Xe} + ^0_{-1}\text{e}$

Gamma (γ) Emission

Gamma emission involves the release of high-energy electromagnetic waves (photons) from a nucleus. Gamma rays have no mass and no charge.

• Symbol: $^0_0\gamma$

• Very high penetration; stopped by thick lead or concrete.

Example:

• $^{60}_{27}\text{Co} \rightarrow ^0_{-1}\text{e} + ^0_0\gamma + ^{60}_{28}\text{Ni}$

Other Radioactive Processes

• Positron Emission: A proton is converted to a neutron and a positron ($^0_{+1}\text{e}$), which is ejected from the nucleus.

• Electron Capture (E.C.): An inner shell electron is captured by the nucleus, combining with a proton to form a neutron.

Artificial Transmutation (Bombardment)

Artificial transmutation occurs when nuclei are bombarded with high-energy particles, resulting in the creation of new nuclei.

• Example: $^{238}_{92}\text{U} + ^4_2\text{He} \rightarrow ^{241}_{94}\text{Pu} + ^1_0\text{n}$

Half-Life of Radioactive Isotopes

Definition and Properties

The half-life of a radioactive isotope is the time required for one-half of a sample to decay. Each isotope has a characteristic half-life, which is constant and unaffected by external conditions.

• Half-life is a statistical measurement.

• Naturally occurring isotopes tend to have long half-lives.

• Medical isotopes often have short half-lives.

Half-Life Calculations

• To determine the amount remaining after a certain time: $N = N_0 \left(\frac{1}{2}\right)^{t/t_{1/2}}$

• Where $N$ = final amount, $N_0$ = initial amount, $t$ = elapsed time, $t_{1/2}$ = half-life.

Example 1: Cesium-137 (half-life = 30 years). How long to decay to 1/16th? 1/16 = (1/2)4 → 4 half-lives → 4 × 30 = 120 years.

Example 2: 2 g sample, half-life = 1 hour. After 3 hours (3 half-lives): 2 g → 1 g → 0.5 g → 0.25 g remains.

Table: Half-Lives of Some Useful Radioisotopes

Radioisotope	Symbol	Radiation	Half-Life	Use
Tritium	$^3_1\text{H}$	$\beta$	12.3 years	Biochemical tracer
Carbon-14	$^{14}_6\text{C}$	$\beta$	5730 years	Archaeological dating
Sodium-24	$^{24}_{11}\text{Na}$	$\beta, \gamma$	14.959 hours	Examining circulation
Phosphorus-32	$^{32}_{15}\text{P}$	$\beta$	14.262 days	Leukemia therapy
Cobalt-60	$^{60}_{27}\text{Co}$	$\beta, \gamma$	5.271 years	Cancer therapy
Uranium-235	$^{235}_{92}\text{U}$	$\alpha$	7.038 × 108 years	Nuclear reactors
Iodine-131	$^{131}_{53}\text{I}$	$\beta, \gamma$	8.021 days	Thyroid therapy

Applications of Half-Life

• Determining how long it takes for a sample to decay to a certain amount.

• Determining how much of a sample remains after a given time.

Isotopic Dating (Carbon-14 Dating)

Carbon-14 dating is used to determine the age of artifacts by measuring the remaining radioactivity. Living organisms maintain a constant level of $^{14}\text{C}$, which decays after death, allowing age estimation.

• Key reaction: $^{14}_7\text{N} + ^1_0\text{n} \rightarrow ^{14}_6\text{C} + ^1_1\text{H}$

Radioactivity in the Environment

Sources of Radioactivity

• Natural sources (e.g., cosmic rays, rocks)

• Energy sources (e.g., nuclear power plants)

• Medical sources (e.g., X-rays, radiotherapy)

• Consumer products (e.g., smoke detectors)

Ionizing Radiation

Definition and Effects

Ionizing radiation is high-energy radiation capable of removing electrons from atoms or molecules, creating ions. These ions are highly reactive and can cause chemical changes in biological tissues.

Symptoms and Biological Effects of Radiation Exposure

• Initial exposure: Often no visible effect.

• Acute symptoms: Nausea, vomiting, diarrhea, hair loss.

• Long-term effects: Genetic mutations, cancers.

• Effects depend on distance, exposure time, radiation type, and location of source.

Penetration of Radiation: Outside vs. Inside the Body

• Outside: Alpha and beta stopped by skin/clothing; gamma penetrates body.

• Inside: Gamma passes through with little damage; alpha and beta cause localized damage.

Protection from Nuclear Radiation

• Shielding (e.g., lead, concrete)

• Increasing distance from source

• Limiting time of exposure

Inverse Square Law for Radiation Intensity:

• $\frac{I_1}{I_2} = \frac{d_2^2}{d_1^2}$

• $I_1$ = intensity at distance $d_1$; $I_2$ = intensity at distance $d_2$

Detection and Measurement of Radiation

Detecting Radiation

• Film Badge: Worn by personnel to monitor exposure.

• Geiger Counter: Detects and measures ionizing radiation.

Measuring Radiation

Unit	Quantity Measured	Description
Curie (Ci)	Decay events	3.7 × 1010 disintegrations per second
Roentgen (R)	Ionizing intensity	Amount of radiation producing 2.1 × 109 charges per cm3 of dry air
Rad	Energy absorbed per gram of tissue	1 rad = 1 R
Rem	Tissue damage	Amount of radiation producing the same damage as 1 rad of X-rays
Sievert (Sv)	Tissue damage	1 Sv = 100 rem

Medical Applications of Radioisotopes

Radioactive Tracers

Radioactive isotopes can be incorporated into molecules and traced by the radiation they emit. This is useful in medical imaging and therapy.

• Medical Imaging: E.g., Iodine-131 for thyroid function tests.

• Radiation Therapy: Targeted radiation to shrink or destroy diseased tissue (e.g., cancer).

Example: Iodine-131 in Medical Imaging

• Radioactive iodine is taken orally and accumulates in the thyroid.

• Hyperthyroidism shows increased uptake of radioactive iodine.

Nuclear Fission and Fusion

Nuclear Fission

In nuclear fission, a large nucleus (such as uranium-235) is bombarded with a neutron, causing it to split into smaller nuclei and release several neutrons and large amounts of energy.

• Fission is the basis for nuclear power and atomic bombs.

• Chain reactions can occur if enough material (critical mass) is present.

Chain Reaction

• A self-sustaining series of fission reactions.

• Critical mass is the minimum amount of material needed to maintain the reaction.

Nuclear Fusion

Nuclear fusion combines small nuclei (such as hydrogen isotopes) into larger nuclei, releasing even more energy than fission. Fusion occurs naturally in stars, including the sun, but requires extremely high temperatures to initiate.

• Fusion is the process powering the sun and hydrogen bombs.

Comparison: Fission vs. Fusion

• Fission: Large nucleus splits; releases energy and neutrons.

• Fusion: Small nuclei combine; releases even more energy; requires high temperature.

Radioactive Isotopes and Stability

Stability of Nuclei

• Nuclear stability depends on the ratio of protons to neutrons.

• Radioactive isotopes have unstable nuclei and emit radiation to become more stable.

Types of Nuclear Radiation

• Alpha (α): 2 protons + 2 neutrons, +2 charge, low penetration.

• Beta (β): High-energy electron, -1 charge, moderate penetration.

• Gamma (γ): High-energy photon, no charge/mass, high penetration.

Penetration and Shielding

• Alpha: Stopped by paper/skin.

• Beta: Stopped by aluminum.

• Gamma: Stopped by thick lead/concrete.