Chapter 5: Nuclear Chemistry and Radioactivity Study Guide

Radioactivity and Nuclear Chemistry

Stable vs Unstable Isotopes

Atoms of the same element can have different numbers of neutrons, resulting in isotopes. The stability of an isotope depends on the ratio of protons to neutrons in its nucleus. Stable isotopes have a balanced ratio, while unstable isotopes undergo radioactive decay to achieve stability.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Stable Isotopes: Do not undergo radioactive decay.

• Unstable Isotopes: Undergo radioactive decay, emitting radiation.

• Example: Magnesium has three common isotopes: 24Mg, 25Mg, and 26Mg.

Classification of Isotopes

Isotopes can be classified as stable or radioactive. Radioactive isotopes emit radiation as they decay.

Element	Stable Isotopes	Radioactive Isotopes
Magnesium	24Mg	23Mg, 27Mg
Iodine	127I	125I, 131I
Uranium	None	235U, 238U

Types of Radioactive Decay

Alpha Decay

Alpha decay occurs when an unstable nucleus emits an alpha particle, which consists of two protons and two neutrons (a helium nucleus). This process decreases the atomic number by 2 and the mass number by 4.

• Alpha Particle: $^{4}_{2}\text{He}$ or $\alpha$

• Example: $^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\text{He}$

Beta Decay

Beta decay occurs when a neutron in the nucleus transforms into a proton and emits a beta particle (an electron). This increases the atomic number by 1, while the mass number remains unchanged.

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

• Example: $^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^{0}_{-1}\text{e}$

Positron Emission

Positron emission occurs when a proton in the nucleus transforms into a neutron and emits a positron (the antimatter equivalent of an electron). This decreases the atomic number by 1, while the mass number remains unchanged.

• Positron: $^{0}_{+1}\text{e}$ or $\beta^+$

• Example: $^{49}_{24}\text{Cr} \rightarrow ^{49}_{25}\text{Mn} + ^{0}_{+1}\text{e}$

Gamma Emission

Gamma emission involves the release of high-energy electromagnetic waves (gamma rays) from the nucleus. Gamma rays do not change the atomic number or mass number, but they release excess energy.

• Gamma Ray: $^{0}_{0}\gamma$ or $\gamma$

Summary of Radiation Types

Different types of radioactive decay produce different particles or waves, each affecting the nucleus in specific ways.

Type	Symbol	Mass Number	Charge
Alpha Particle	$^{4}_{2}\text{He}$	4	2+
Beta Particle	$^{0}_{-1}\text{e}$	0	1−
Positron	$^{0}_{+1}\text{e}$	0	1+
Gamma Ray	$^{0}_{0}\gamma$	0	0
Proton	$^{1}_{1}\text{H}$	1	1+
Neutron	$^{1}_{0}\text{n}$	1	0

Penetration Ability of Radiation

Comparing Radiation Types

The ability of radiation to penetrate materials varies by type. Alpha particles are stopped by paper, beta particles by metal, neutrons by water, and gamma rays require dense materials like lead for shielding.

• Alpha: Least penetrating, stopped by paper.

• Beta: Moderate penetration, stopped by metal.

• Gamma: Most penetrating, requires lead or concrete.

Measurement of Radiation

Units of Measurement

Radiation is measured in terms of activity, absorbed dose, and biological damage. Common units include curie (Ci), rad, and rem, while SI units are becquerel (Bq), gray (Gy), and sievert (Sv).

Measurement	Common Unit	SI Unit	Relationship
Activity	curie (Ci)	becquerel (Bq)	1 Ci = 3.7 × 1010 Bq
Absorbed Dose	rad	gray (Gy)	1 Gy = 100 rad
Biological Damage	rem	sievert (Sv)	1 Sv = 100 rem

Sources and Effects of Radiation

Common Radiation Sources

Radiation exposure comes from both natural and artificial sources. The average annual dose for Americans is about 3.6 mSv.

Source	Dose (mSv)
Ground	0.2
Air, water, food	0.3
Cosmic rays	0.4
Wood, concrete, brick	0.5
Chest X-ray	0.2
Dental X-ray	0.2
Mammogram	0.4
Hip X-ray	0.6
Lumbar spine X-ray	0.7
Upper GI X-ray	0.6
Nuclear power plants	0.001
Television	0.2
Air travel	0.1
Radon	2*

Toxic Radiation Exposure

High doses of radiation can cause severe biological effects, including radiation sickness and death. The LD50 (lethal dose for 50% of population) varies by organism.

Life Form	LD50 (Sv)
Insect	1000
Bacterium	500
Rat	8
Human	5
Dog	3

Nuclear Fission and Fusion

Chain Reactions (Fission)

Nuclear fission is the splitting of a heavy nucleus into lighter nuclei, releasing energy and neutrons. These neutrons can cause further fission reactions, resulting in a chain reaction.

• Example: $^{235}_{92}\text{U} + ^{1}_{0}\text{n} \rightarrow ^{140}_{56}\text{Ba} + ^{94}_{36}\text{Kr} + 3 ^{1}_{0}\text{n}$

Nuclear Power Reactors (Fission)

Nuclear reactors use controlled fission reactions to generate heat, which is used to produce steam and drive turbines for electricity generation.

Fusion

Nuclear fusion is the process where two light nuclei combine to form a heavier nucleus, releasing large amounts of energy. Fusion is the energy source of stars, including the Sun.

• Example: $^{2}_{1}\text{H} + ^{3}_{1}\text{H} \rightarrow ^{4}_{2}\text{He} + ^{1}_{0}\text{n} + \text{energy}$

Half-Life and Radioactive Decay

Half-Life

The half-life of a radioactive isotope is the time required for half of the sample to decay. This property is used to estimate the age of materials and to understand the persistence of radioactivity.

• Formula: $N = N_0 \left( \frac{1}{2} \right)^{\frac{t}{t_{1/2}}}$

• Example: Iodine-131 has a half-life of 8 days.

Half-Lives of Isotopes

Different isotopes have different half-lives, ranging from seconds to millions of years. This affects their use in medicine, dating, and environmental studies.

Element	Radioisotope	Half-Life	Type of Radiation
Carbon	$^{14}\text{C}$	5730 yr	Beta
Potassium	$^{40}\text{K}$	1.3 × 109 yr	Beta, gamma
Radium	$^{226}\text{Ra}$	1600 yr	Alpha
Strontium	$^{90}\text{Sr}$	28.8 yr	Beta
Uranium	$^{238}\text{U}$	4.5 × 109 yr	Alpha

Carbon Dating (Beta Decay)

Carbon dating uses the decay of carbon-14 to estimate the age of organic materials. Living organisms constantly exchange carbon with the environment, but after death, the carbon-14 decays at a known rate.

• Formula: $^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^{0}_{-1}\text{e}$

Medical Applications of Radioactivity

Radiotracers and PET Scans

Radioactive isotopes are used in medicine for diagnosis and treatment. Radiotracers help visualize biological processes, and positron emission tomography (PET) scans use positron-emitting isotopes to image organs and tissues.

Isotope	Half-Life	Radiation	Medical Application
Au-198	2.7 days	Beta	Liver imaging; treatment of abdominal carcinoma
Ce-141	32.5 days	Beta	Gastrointestinal tract diagnosis
C-11	20 min	Positron	PET imaging
I-131	8.0 days	Beta	Treatment of thyroid, brain, and prostate cancer
Xe-133	5.2 days	Beta	Pulmonary function diagnosis

Radiation Medical Applications

Radioisotopes are used for imaging, cancer treatment, and monitoring organ function. The choice of isotope depends on its half-life and type of radiation emitted.

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