Chapter 20: Radioactivity and Nuclear Chemistry – Structured Study Notes

Radioactivity and Nuclear Chemistry

Introduction

This chapter explores the fundamental principles of radioactivity and nuclear chemistry, focusing on the properties of radioactive materials, types of radioactive decay, nuclear equations, and the detection of radioactivity. These concepts are essential for understanding the behavior of unstable nuclei and their applications in science and technology.

Properties of Radioactivity

Ionization and Penetration

• Radioactive rays can ionize matter, causing uncharged matter to become charged. This property is the basis for devices such as the Geiger counter and electroscope.

• Radioactive rays possess high energy, can penetrate matter, and can cause phosphorescent chemicals to glow.

• Example: The green glow in certain toys is due to phosphorescence caused by radioactive rays.

Radioactivity

Unstable Elements

• Elements with atomic numbers greater than 84 (e.g., plutonium, radium, uranium) are inherently unstable and continuously emit radiation.

• This emission leads to the transformation of atoms into new elements, called daughter nuclei.

• Radioactivity is a natural, irreversible process that occurs continuously.

Becquerel's Experiment

Discovery of Radioactivity

• Antoine-Henri Becquerel investigated whether phosphorescent minerals emitted X-rays.

• Phosphorescence is the long-lived emission of light by atoms or molecules after absorbing light.

• X-rays are detected by their ability to penetrate matter and expose photographic plates.

Facts about the Nucleus

Nuclear Structure

• The nucleus is very small compared to the atom's volume but contains nearly all its mass and is extremely dense.

• Composed of protons and neutrons (collectively called nucleons).

• Each atom of an element has the same number of protons (atomic number, Z), but may have different numbers of neutrons (isotopes).

Isotopes

Identification and Symbolism

• Isotopes are identified by their mass number (A): $A = \text{number of protons} + \text{number of neutrons}$

• Isotopic symbol: $\frac{A}{Z}X$

• The nucleus of an isotope is called a nuclide. Most nuclides are radioactive (radionuclides).

Types of Radioactive Decay

Decay Modes

• Alpha (α) particles: Charge +2, mass 4 amu, helium nucleus.

• Beta (β) particles: Charge -1, negligible mass, electron-like.

• Gamma (γ) rays: High-energy photons, no mass or charge.

• Some nuclei emit positrons (positively charged electrons) or undergo electron capture (an inner electron is pulled into the nucleus).

Strong Nuclear Force

Binding and Stability

• The strong nuclear force binds protons and neutrons in the nucleus, overcoming electrostatic repulsion between protons.

• Neutrons contribute to stability by adding to the strong force without causing repulsion.

Alpha (α) Decay

Mechanism and Effects

• An alpha particle is a $^4_2\text{He}$ nucleus.

• Alpha decay: unstable nucleus emits two protons and two neutrons.

• Most ionizing, least penetrating (stopped by paper or cloth).

• Atomic number decreases by 2; mass number decreases by 4.

Beta (β) Decay

Mechanism and Effects

• A beta particle is an electron emitted when a neutron converts to a proton and an electron.

• Equation: $^1_0\text{neutron} \rightarrow ^0_{-1}\beta + ^1_1\text{proton}$

• Beta decay: atomic number increases by 1; mass number unchanged.

• More penetrating than alpha, but less ionizing (stopped by heavy cloth).

Gamma (γ) Emission

Mechanism and Effects

• Gamma rays are high-energy photons emitted from the nucleus.

• No change in atomic or mass number.

• Least ionizing, most penetrating (requires lead or thick cement for protection).

Positron Emission

Mechanism and Effects

• A positron ($^0_{+1}e$) has a charge of +1 and negligible mass (antielectron).

• Formed when a proton converts to a neutron and a positron: $^1_1\text{proton} \rightarrow ^0_{+1}e + ^1_0\text{neutron}$

• Atomic number decreases by 1; mass number unchanged.

Electron Capture

Mechanism and Effects

• An inner electron is captured by the nucleus and combines with a proton to form a neutron: $^1_1\text{proton} + ^0_{-1}e \rightarrow ^1_0\text{neutron}$

• Atomic number decreases by 1; mass number unchanged.

Table: Modes of Radioactive Decay

Comparison of Decay Modes

Decay Mode	Process	Change in Z	Change in A	Example
Alpha	Parent nuclide → Daughter nuclide + α particle	-2	-4	$^{238}_{92}U \rightarrow ^{234}_{90}Th + ^4_2He$
Beta	Neutron → Proton + β particle	+1	0	$^{137}_{55}Cs \rightarrow ^{137}_{56}Ba + ^0_{-1}e$
Gamma	Rearrangement of nucleus, emission of γ ray	0	0	$^{60}_{27}Co^* \rightarrow ^{60}_{27}Co + \gamma$
Positron	Proton → Neutron + positron	-1	0	$^{30}_{15}P \rightarrow ^{30}_{14}Si + ^0_{+1}e$
Electron Capture	Proton + electron → Neutron	-1	0	$^{111}_{50}Sn + ^0_{-1}e \rightarrow ^{111}_{49}In$

Penetration of Radioactive Particles

Protection from Radiation

• Alpha particles: stopped by paper or skin.

• Beta particles: stopped by aluminum or heavy cloth.

• Gamma rays: require concrete or lead for shielding.

Nuclear Equations

Conservation Laws

• Nuclear processes are described by nuclear equations.

• Atomic numbers and mass numbers are conserved: the sum on both sides must be equal.

• Example: $^{238}_{92}U \rightarrow ^{234}_{90}Th + ^4_2He$

Practice Problems

Writing Nuclear Equations

• Alpha decay: $^{224}_{88}Ra \rightarrow ^{220}_{86}Rn + ^4_2He$

• Beta decay: $^{249}_{98}Cf \rightarrow ^{249}_{99}Es + ^0_{-1}e$

• Positron emission: $^{15}_8O \rightarrow ^{15}_7N + ^0_{+1}e$

• Electron capture: $^{111}_{50}Sn + ^0_{-1}e \rightarrow ^{111}_{49}In$

What Causes Nuclei to Decompose?

Strong Force and Stability

• The strong nuclear force holds nucleons together, acting only over very short distances.

• Neutrons stabilize the nucleus by contributing to the strong force without causing repulsion.

N/Z (Neutrons/Protons) Ratio

Stability of Nuclei

• The N/Z ratio is crucial for nuclear stability.

• For Z = 1–20, stable N/Z ≈ 1; for Z = 20–40, stable N/Z ≈ 1.25; for Z = 40–80, stable N/Z ≈ 1.5; for Z > 83, no stable nuclei exist.

• If N/Z is too high, beta decay occurs; if too low, positron emission or electron capture occurs.

Magic Numbers & Stable Nuclei

Stability Factors

• Stability is affected by the actual numbers of protons and neutrons.

• Most stable nuclei have even numbers of both protons and neutrons.

• Nuclei with "magic numbers" (2, 8, 20, 28, 50, 82, 126) are especially stable.

Decay Series

Sequential Decay

• Radioactive nuclides often decay into other radioactive nuclides, forming a decay series until a stable nuclide is produced.

• All atoms with Z > 83 are radioactive and participate in decay series.

Natural Radioactivity

Background Radiation

• Radioactive minerals are present in air, ground, water, and even food.

• Exposure from natural sources is called background radiation.

Detecting Radioactivity

Detection Methods

• Radioactive particles have high energy and can be detected by various methods:

• Photographic film (film-badge dosimeters) records exposure.

• Electroscope detects ionization of air.

• Geiger-Müller counter counts electrons from ionized argon gas.

• Scintillation counter counts flashes of light from certain chemicals.