Atoms and Radioactivity

2.1 Atoms and Their Components

Atoms are the fundamental building blocks of matter, composed of three main subatomic particles: protons, neutrons, and electrons. Understanding their arrangement and properties is essential for grasping atomic structure and chemical behavior.

• Protons: Positively charged particles located in the nucleus. Each proton has a charge of +1 and a mass of approximately 1 atomic mass unit (amu).

• Neutrons: Neutral particles (no charge) also found in the nucleus, with a mass close to 1 amu.

• Electrons: Negatively charged particles found in the electron cloud surrounding the nucleus. Their mass is about 1/2000 that of a proton or neutron and is considered negligible in atomic mass calculations.

• Nucleus: The dense central core of the atom, containing protons and neutrons.

• Electron Cloud: The region around the nucleus where electrons are likely to be found. Most of the atom's volume is empty space occupied by this cloud.

Atomic Mass Unit (amu): Defined as one-twelfth the mass of a carbon-12 atom. Protons and neutrons each have a mass of 1 amu; electrons are considered to have 0 amu for most calculations.

2.2 Atomic Number and Mass Number

The identity and properties of an atom are determined by its atomic number and mass number. These values are fundamental for distinguishing elements and isotopes.

• Atomic Number (Z): The number of protons in an atom, unique to each element. It also equals the number of electrons in a neutral atom.

• Mass Number (A): The total number of protons and neutrons in an atom's nucleus.

Formula:

• Symbolic Notation: Atoms are often represented as , where X is the element symbol, A is the mass number, and Z is the atomic number.

Steps to Determine Subatomic Particles:

1. Find the element on the periodic table.

2. Use the atomic number to determine protons and electrons.

3. Subtract atomic number from mass number to find neutrons.

4. Write the symbolic notation.

2.3 Isotopes and Atomic Mass

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers. The atomic mass of an element is a weighted average of all its naturally occurring isotopes.

• Isotopes: Atoms with the same number of protons but different numbers of neutrons (e.g., carbon-12, carbon-13, carbon-14).

• Atomic Mass: The weighted average mass of all isotopes of an element, as found on the periodic table.

Example: Carbon has isotopes carbon-12, carbon-13, and carbon-14. The atomic mass listed on the periodic table reflects the average based on their natural abundance.

2.4 Radioactivity and Radioisotopes

Some isotopes are unstable and spontaneously emit energy from their nuclei, a phenomenon known as radioactivity. These unstable isotopes are called radioisotopes.

• Radioactivity: The spontaneous emission of energy from the nucleus of an unstable atom.

• Radioisotopes: Isotopes that emit radiation to become more stable.

• Radioactive Decay: The process by which unstable nuclei emit radiation to achieve stability.

Types of Radiation:

• Alpha (α) particles: Helium nuclei (2 protons, 2 neutrons), 2+ charge, low penetration.

• Beta (β) particles: High-energy electrons, 1- charge, moderate penetration.

• Gamma (γ) rays: High-energy electromagnetic radiation, no charge, high penetration.

• Positrons: Same mass as beta particles but with a positive charge.

• Neutrons: No charge, can induce radioactivity when absorbed by nuclei.

Biological Effects: Ionizing radiation can damage living cells by ejecting electrons, potentially causing cancer. Protective measures include shielding and monitoring exposure.

Radiation Measurement: The sievert (Sv) measures biological damage; the average annual dose is about 6.2 millisieverts (mSv).

2.5 Nuclear Equations and Radioactive Decay

Nuclear equations represent the changes in atomic nuclei during radioactive decay. The sum of atomic and mass numbers must be conserved in these equations.

• Alpha Decay: Emission of an alpha particle decreases atomic number by 2 and mass number by 4.

• Beta Decay: Emission of a beta particle increases atomic number by 1 (neutron becomes a proton).

• Gamma Decay: Emission of gamma rays does not change atomic or mass numbers.

• Positron Emission: Emission of a positron decreases atomic number by 1 (proton becomes a neutron).

Steps for Writing Nuclear Equations:

1. Write the symbolic notation for the decaying isotope.

2. Place the emitted radiation on the product side.

3. Balance atomic and mass numbers to determine the new isotope.

Production of Radioisotopes: Stable isotopes can be converted to radioisotopes by bombarding them with particles (e.g., neutrons, protons).

2.6 Radiation Units and Half-Lives

The activity of a radioactive sample is measured by the number of disintegrations per second. The rate of decay is characterized by the half-life, the time required for half the atoms in a sample to decay.

• Curie (Ci): Traditional unit for radioactivity (disintegrations per second).

• Becquerel (Bq): SI unit for radioactivity (1 disintegration per second).

• Half-Life (t1/2): Time required for half of the radioactive atoms to decay.

• Biological Half-Life: Time required for half of a substance to be eliminated from the body.

• Effective Half-Life: Combination of physical and biological half-lives, relevant for medical isotopes.

Half-Life Calculation Formula:

where is the number of half-lives elapsed.

2.7 Medical Applications for Radioisotopes

Radioisotopes are widely used in medicine for both diagnosis and treatment. Their ability to concentrate in specific tissues and emit detectable radiation makes them valuable tools in healthcare.

• Diagnostic Use: Small amounts of radioisotopes (tracers) with short half-lives are used in imaging to detect abnormalities (e.g., Technetium-99m for lung scans).

• Therapeutic Use: Higher doses of radioisotopes (e.g., Iodine-131 for thyroid cancer) are used to destroy diseased tissues.

• External Beam Radiation Therapy: Gamma rays from sources like cobalt-60 are directed at tumors.

• Brachytherapy: Radioactive seeds are implanted directly into tumors.

• Positron Emission Tomography (PET): Uses isotopes like Fluorine-18 to detect functional abnormalities in organs, especially the brain.

Safety: Minimizing exposure time and using the lowest effective dose are essential to reduce risks associated with radiation.