Radioactivity Revision Notes

Atomic Structure and Charge

Atoms are neutral, with equal numbers of protons (+) and electrons (-). Protons are in the nucleus; electrons orbit in shells. The outermost electrons are valence electrons, key to chemical bonding and ion formation.

How Positive Ions Form

Positive ions (cations) form when an atom loses one or more outer electrons. Losing negative charges means the number of protons (positive) now exceeds the number of electrons (negative), resulting in a net positive charge. The nucleus does not change.

Energy Requirements

Energy is needed to remove an electron from an atom. The electrostatic attraction between the positively charged nucleus and negatively charged electrons must be overcome for ionization to occur.

Stability and Electron Loss

Atoms often lose electrons to achieve a stable electron configuration, typically a full outer electron shell, which is energetically favourable. This tendency drives the formation of many positive ions.

Radioactive Decay Fundamentals

Unstable atomic nuclei release energy and particles, known as radiation, to become more stable. This process is called radioactive decay. It's a spontaneous and random event, meaning that while the half-life of a radioactive isotope is constant, you cannot predict exactly when any single nucleus will decay.

Types of Radiation Emitted

Common forms of radiation emitted by unstable nuclei include: alpha particles (composed of two protons and two neutrons), beta-minus particles (high-energy electrons), beta-plus particles (positrons, which are anti-electrons), gamma rays (high-energy electromagnetic waves), and neutrons. Each type varies in its characteristics, such as mass, charge, and energy.

Radioactive Decay and Nuclear Excitation

After an unstable nucleus undergoes alpha or beta decay, the resulting daughter nucleus is frequently in an excited state. This means it possesses excess internal energy.

Nuclear Rearrangement and Energy Loss

To return to a more stable, lower-energy state, the excited nucleus undergoes a nuclear rearrangement. During this process, the excess energy is released.

Gamma Radiation (γ-rays)

The energy released during nuclear rearrangement is emitted as gamma radiation. Gamma rays are high-energy electromagnetic waves. Their emission does not change the type of element (atomic number) or the mass number, but it reduces the nucleus's energy.

Terrestrial Sources of Background Radiation

Background radiation from Earth, or terrestrial radiation, primarily originates from radioactive isotopes found in the Earth's crust. Key examples include uranium, thorium, and potassium-40. These elements are present in varying concentrations in rocks, soil, and water. A significant contributor is radon gas, which is a decay product of uranium and thorium. Radon can accumulate in enclosed spaces like homes, particularly in areas with underlying granite, which is rich in these radioactive elements. Other materials used in construction, such as bricks, concrete, and plaster, also contain trace amounts of these naturally occurring radioactive substances, adding to the terrestrial background radiation.

Cosmic Sources of Background Radiation

Radiation from space, known as cosmic radiation, is composed of high-energy particles from the Sun and distant stars. These primary cosmic rays interact with atoms and molecules in Earth's atmosphere, leading to the creation of secondary radiation, including muons, electrons, and photons. The Earth's atmosphere acts as a shield, reducing the intensity of this radiation at sea level. However, the dose of cosmic radiation increases significantly with altitude (e.g., during air travel) because there is less atmospheric shielding. The Earth's magnetic field also plays a role, deflecting some of these charged particles, with stronger deflection near the equator compared to the poles, meaning cosmic radiation levels vary with latitude.

Overall Contributions to Background Radiation

Natural sources account for about 85% of an average person's annual radiation dose. Terrestrial and cosmic radiation are the main natural components. The remaining percentage comes from human-made sources like medical X-rays, nuclear power, and other industrial applications. The exact proportion of terrestrial versus cosmic radiation varies geographically; for instance, mountainous regions experience higher cosmic radiation, while areas with high uranium concentrations in the ground experience higher terrestrial radiation.