Nuclear Chemistry: Radioactivity and Decay

Learning Objectives

Define radioactivity and identify the three main types of decay (alpha, beta, gamma).

Write and balance nuclear decay equations.

Define half-life and perform calculations related to it.

Distinguish between nuclear fission and fusion.

The Unstable Nucleus

Nuclear chemistry is the study of reactions that involve changes in atomic nuclei. This is distinct from chemical reactions, which only involve the rearrangement of electrons.

Radioactivity

Radioactivity is the spontaneous emission of particles or energy (radiation) from an unstable atomic nucleus.

Alpha (α) Decay: Emission of an alpha particle (a helium nucleus, ⁴₂He).

Effect: Mass number decreases by 4, atomic number decreases by 2.

Beta (β) Decay: A neutron converts to a proton, and a high-energy electron (⁰₋₁e) is ejected.

Effect: Mass number is unchanged, atomic number increases by 1.

Gamma (γ) Decay: Emission of a gamma ray (a high-energy photon).

Effect: No change in mass number or atomic number; the nucleus simply moves to a lower energy state.

Balancing Nuclear Equations

In a balanced nuclear equation, the sum of the mass numbers and the sum of the atomic numbers must be equal on both sides of the reaction.

Example: Alpha decay of Uranium-238

²³⁸₉₂U → ⁴₂He + ²³⁴₉₀Th

Mass numbers: 238 = 4 + 234

Atomic numbers: 92 = 2 + 90

Half-Life

The half-life (t₁/₂) is the time required for half of the radioactive nuclei in a sample to undergo decay. This is a constant, first-order kinetic process.

Calculation: Amount Remaining = Initial Amount (1/2)ⁿ, where n* is the number of half-lives that have passed.

Fission vs. Fusion

Nuclear Fission: The splitting of a single large, heavy nucleus into two or more smaller nuclei, releasing a large amount of energy and neutrons. This is the process used in nuclear power plants.

Nuclear Fusion: The combining of two small, light nuclei to form a single, heavier nucleus, releasing an even greater amount of energy per nucleon. This is the process that powers the sun.

Key Terms

Radioactivity: The emission of ionizing radiation or particles caused by the spontaneous disintegration of atomic nuclei.
Half-life: The time required for a quantity of a radioactive isotope to reduce to half of its initial value.
Alpha Particle: A particle consisting of two protons and two neutrons, equivalent to a helium nucleus, emitted during alpha decay.
Beta Particle: A high-speed electron or positron emitted in the radioactive decay of an atomic nucleus.
Nuclear Fission: The splitting of a heavy atomic nucleus into two or more smaller nuclei, accompanied by the release of a large amount of energy.
Nuclear Fusion: The process by which two or more light atomic nuclei combine to form a single heavier nucleus, accompanied by the release of a large amount of energy.