Radioactivity is the process by which unstable atomic nuclei lose energy by emitting radiation. As an honors chemistry student, you should be familiar with various types of radioactive decay and nuclear reactions that can alter the composition of a nucleus.

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🚀 Types of Radioactivity

Alpha Decay (α-decay)

What is an alpha particle?

• An alpha particle consists of two protons and two neutrons, making it identical to a helium-4 nucleus.
• Alpha emission typically occurs in heavy elements like uranium or radium.

How does it interact with matter?

• Alpha particles have low penetration ability due to their size and double positive charge. They can be stopped by a sheet of paper or even the skin.

Atomic changes:

• When an alpha particle is emitted from a nucleus, the mass number decreases by four units and the atomic number decreases by two units.

Image Courtesy of Nuceng

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Beta Decay

Beta-minus Decay (β⁻-decay)

What happens during β⁻-decay?

• A neutron in an unstable nucleus is transformed into a proton while emitting an electron (beta particle) and an antineutrino.
• This process increases the atomic number by one while leaving the mass number unchanged.

Beta-plus Decay (β⁺-decay or Positron Emission)

What happens during β⁺-decay?

• A proton in an unstable nucleus converts into a neutron, releasing a positron (the electron's antimatter counterpart) and a neutrino.
• The atomic number decreases by one, but again, the mass number remains constant.

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Gamma Decay (γ-decay)

• Gamma rays are high-energy photons emitted from a nucleus as it transitions from a higher energy state to a lower one.
• This type of decay does not change either the mass number or the atomic number but stabilizes the nucleus after other types of decay events.

Image Courtesy of Wizeprep

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Electron Capture

• An inner orbital electron is captured by the nucleus where it combines with a proton to form a neutron and emit a neutrino.
• Electron capture decreases the atomic number by one without altering the mass number.

Neutron Emission

Some extremely neutron-rich nuclei may achieve stability through neutron emission, where one or more neutrons are ejected from the nucleus.

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⚛️ Nuclear Reactions vs. Chemical Reactions

Nuclear reactions involve changes in an atom's nucleus, unlike chemical reactions that involve electrons orbiting outside the nucleus. The key differences include:

Fission

• Splitting heavy nuclei into smaller ones releases vast amounts of energy often used in nuclear power plants or weapons.

Fusion

• Lighter nuclei fuse together at high temperatures and pressures to form heavier nuclei, releasing energy as occurs in stars like our sun.

Image Courtesy of Department of Energy

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🏗️ Stability and Decay Factors

Understanding what makes certain isotopes more stable than others involves considering:

Nuclear Binding Energy

The stronger binding energies per nucleon equate to more stable nuclei; hence isotopes far from this optimum value tend to undergo radioactive decay to reach stability.

Magic Numbers

Proton or neutron numbers such as 2, 8, 20 indicate unusually stable configurations due to closed shells within nuclear structure models.

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🔬 External Factors Influencing Stability

Conditions like temperature and pressure can affect nuclear reactions; for instance:

Stellar Nucleosynthesis

In stars where extreme conditions prevail, nuclear synthesis produces heavier elements through fusion processes under immense temperatures and pressures not found on Earth.

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❓Radioactivity & Nuclear Reactions Practice Questions

1. A sample of americium-241 undergoes alpha decay. What are the products of this reaction?

Explanation: When americium-241 undergoes alpha decay, it emits an alpha particle, which is a helium-4 nucleus. The resulting daughter nucleus and atomic number can be determined by subtracting 2 from the atomic number of the parent nucleus and subtracting 4 from the mass number.

For americium-241:

• Atomic number (Z): 95
• Mass number (A): 241

After emitting an alpha particle:

• Atomic number: 95−2=93
• Mass number: 241−4=237

Answer: The products of the reaction are a neptunium-237 nucleus and an alpha particle.

1. Describe how beta decay affects the stability of a nucleus. Consider both β⁻ and β⁺ decays.

Explanation: β⁻ decay often occurs in neutron-rich nuclei. By converting a neutron into a proton, the nucleus moves towards a more stable configuration in terms of the neutron-to-proton ratio, which tends to increase the stability of the nucleus. β⁺ decay often occurs in proton-rich nuclei. By converting a proton into a neutron, the nucleus moves towards a more stable configuration in terms of the neutron-to-proton ratio, which tends to increase the stability of the nucleus.

1. Explain why neutron emission might occur more frequently in elements that lie above the band of stability on the chart of nuclides.

Explanation: Nuclei above the band of stability typically have an excess of neutrons compared to the number of protons. This imbalance in the neutron-to-proton ratio can lead to instability within the nucleus. In addition, as the number of protons increases within a nucleus, the electromagnetic repulsion between positively charged protons also increases. This repulsion tends to destabilize the nucleus.

1. Why do we say that elements with magic numbers are especially stable?

Explanation: Magic numbers are specific values of protons or neutrons that result in filled nuclear shells, similar to the concept of electron shells in atomic orbitals. Elements with "magic numbers" of protons or neutrons are considered especially stable due to the unique arrangement of nucleons within their nuclei, which leads to enhanced nuclear stability.

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📚 Use this study guide as your starting point for understanding radioactivity and nuclear reactions. Remember to do practice questions! Good luck!