Nuclear Chemistry – Radiation and the Band of Stability

This topic is part of Year 11 HSC Chemistry course under the topic of Atomic Structure and Atomic Mass.

HSC Chemistry Syllabus

Investigate the properties of unstable isotopes using natural and human-made radioisotopes as examples including but not limited to:

– Types of radiation

– Types of balanced nuclear reactions

Radiation & Nuclear Reactions

This video will explore different types of radiation and demonstrate how to write and balance nuclear reactions. Furthermore this video will briefly discuss radioisotopes and explain how the band of stability can be used to predict the radioactivity of isotopes.

Radioisotopes

Unstable isotopes emit radiation, and therefore are known as radioisotopes. In fact, the vast majority of isotopes that exist in nature are unstable. By emitting radiation, an unstable isotope will change either its nuclear composition (protons and neutrons) or energy state.

Isotopes of carbon, such as carbon-12, carbon-13, and carbon-14, have varying numbers of neutrons, which affects their stability. Carbon-12 and carbon-13 are stable, non-radioactive isotopes. Carbon-14, however, is radioactive and known for its use in radiocarbon dating.

Radioactive isotopes, or radioisotopes, emit radiation detectable by instruments like Geiger counters or by photographic film methods. While it's true that all elements with an atomic number greater than 82 (lead, with atomic number 82, is usually considered the last stable element) are radioactive, including uranium. For instance, rubidium and rhenium have both stable and radioactive isotopes. Specifically, rubidium-87 is radioactive, widely used in radiometric dating.

Types of Radiation

There are three types of radiation: Alpha (`\alpha`), Beta (`\beta`), and Gamma (`\gamma`). In HSC Chemistry, beta decay typically refers to beta-minus decay.

For information on beta-plus decay, click here.

Property	Alpha (α)	Beta-minus (β)	Gamma (ɣ)
Component	Helium nucleus	Electron	Electromagnetic Radiation
Nuclear Symbol	$$^4_2\alpha \text{ or } ^4_2\text{He}$$	$$^0_{-1}\beta $$	$$^0_0\gamma $$
Electric charge	+2	-1	0
Penetration	Poor penetration Stopped by thin paper	Medium/low penetration Stopped by 5mm of aluminium	Very strong penetration can travel through 2 inches of lead

Nuclear Reactions and Equations

Nuclear chemistry explores the processes that alter the composition, structure, or energy of atomic nuclei. The term "nuclide" refers to distinct types of atomic nuclei, characterised by a specific number of protons and neutrons. Nuclides are typically denoted by the element's name, followed by the mass number (the total number of protons and neutrons). For example, "carbon-14" describes a carbon nuclide with 6 protons and 8 neutrons, totaling a mass number of 14.

Radioactive decay, a key focus within nuclear chemistry, describes the transformation of an unstable nucleus (parent nucleus) into a more stable nucleus (daughter nucleus) by emitting radiation. This process naturally tends toward the creation of energetically more favourable, lower mass daughter nuclei

The transformation can be represented as:

Parent Nucleus `\rightarrow` Radiation + Daughter Nucleus

Like chemical equations where the number and types of atoms are accounted for to present a balanced chemical equation, nuclear decay equations can also be balanced. this involves accounting for the total number of protons (which determines the element) and neutrons in the decay process. The conservation of nucleons (protons and neutrons) and the overall energy are fundamental to writing balanced nuclear equations.

Example of Alpha Decay

Parent nucleus	$$\rightarrow$$	Radiation	+	Daughter nucleus
$$^A_ZX$$	$$\rightarrow$$	$$^4_2\alpha$$	+	$$^{A-4}_{Z-2}X$$
Radium-266	$$\rightarrow$$	Alpha Particle	+	Radon-222
$$^{266}_{88}Ra$$	$$\rightarrow$$	$$^4_2\alpha$$	+	$$^{262}_{86}Ra$$

Example of Beta-minus Decay

Parent nucleus	$$\rightarrow$$	Radiation	+	Daughter nucleus
$$^A_ZX$$	$$\rightarrow$$	$$^0_{-1}\beta$$	+	$$^{A}_{Z+1}X$$
Iodine-131	$$\rightarrow$$	Beta Particle	+	Xenon-131
$$^{131}_{53}I$$	$$\rightarrow$$	$$^0_{-1}\beta$$	+	$$^{131}_{54}Ra$$

Band of Stability

When isotopes are plotted on a graph with axes 'number of neutrons' and 'number of protons', the region outlining the stable isotopes is referred to as the band of stability.

The concept of a "band of stability" explains why certain nuclides are stable and others are not. The nuclear composition or neutron-to-proton ratio of an isotope is a critical factor for its stability. This is discussed in greater detail here.

The band of stability concludes at lead (Pb), element 82, beyond which all elements are radioactive. This includes bismuth (Bi, atomic number 83), such as the isotope bismuth-209.

Larger atomic nuclei require more neutrons to stabilise due to increased proton-proton repulsion within the nucleus. These protons which are positively charged, repel according to the well-known rule “opposites attract, similar repel”. Thus, to overcome this issue, the nuclei require larger numbers of neutrons to provide compensating strong forces to overcome these ‘electrostatic repulsions’ and hold the nucleus together.

When Does Each Type of Radioactive Decay Occur?

The type of radioactive decay a radionuclide undergoes is largely determined by its neutron-to-proton ratio in relation to the ideal ratios within the band of stability, as well as its overall position on the nuclear chart. There are primarily two scenarios where a nuclide may undergo decay to achieve greater stability:

Heavy Nuclei (Atomic Number ≥ 83) – Alpha Decay

Nuclides with atomic numbers greater than or equal to 83, positioned beyond the upper right edge of the band of stability are inherently unstable due to their large number of protons and neutrons. These heavy nuclei undergo alpha decay to shed mass and move diagonally towards a more stable configuration. In alpha decay, an alpha particle (a helium nucleus) which is composed of two protons and two neutrons is emitted

Neutron-rich Nuclei (High n/p Ratio) – Beta Decay

Nuclides with a neutron-to-proton ratio higher than the optimal range for stability are considered neutron-rich. These nuclei undergo beta decay, where a neutron in the nucleus is transformed into a proton. During this process, the neutron emits an electron (referred to as a beta particle) and an antineutrino, effectively converting into a proton and slightly increasing the atomic number without changing the mass number.

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