BackAtoms and Radioactivity: Structure, Isotopes, and Medical Applications
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Atoms and Their Components
Subatomic Particles and Atomic Structure
Atoms are the fundamental units of matter, composed of three main subatomic particles: protons, neutrons, and electrons. The arrangement and properties of these particles determine the identity and behavior of each element.
Protons: Positively charged particles located in the nucleus; define the atomic number and identity of the element.
Neutrons: Neutral particles also found in the nucleus; contribute to the atom's mass but not its charge.
Electrons: Negatively charged particles found in the electron cloud surrounding the nucleus; their number equals the number of protons in a neutral atom.
The nucleus contains most of the atom's mass, while the electron cloud occupies most of its volume. The mass of an electron is about 1/2000 that of a proton or neutron and is often considered negligible in atomic mass calculations.
Relative Mass and Charge of Subatomic Particles
Subatomic Particle | Symbol | Charge | Relative Mass | Location |
|---|---|---|---|---|
Electron | e- | 1- | 0.0005 (1/2000) | Outside nucleus |
Proton | p | 1+ | 1 | Nucleus |
Neutron | n | 0 | 1 | Nucleus |
Atomic Number and Mass Number
Defining Elements and Calculating Subatomic Particles
The atomic number (Z) is the number of protons in an atom and determines the element's identity. The mass number (A) is the sum of protons and neutrons in the nucleus. Atoms of the same element always have the same number of protons, but may have different numbers of neutrons.
Atomic Number (Z): Number of protons; found above the element symbol on the periodic table.
Mass Number (A): Number of protons + number of neutrons.
Symbolic Notation: , where X is the element symbol.
Isotopes and Atomic Mass
Isotopes and Their Representation
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers. Isotopes are represented either by symbolic notation or by stating the mass number after the element name (e.g., carbon-12, carbon-13, carbon-14).
Atomic Mass: The weighted average of all naturally occurring isotopes of an element, shown on the periodic table below the element symbol.
Example: Chlorine has two main isotopes, chlorine-35 and chlorine-37. The atomic mass of chlorine (35.45 amu) indicates that chlorine-35 is more abundant.
Radioactivity and Radioisotopes
Types of Radiation and Their Properties
Radioactivity is the spontaneous emission of energy from the nucleus of unstable atoms (radioisotopes). The main types of nuclear radiation are:
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: Positive electrons, 1+ charge.
Neutrons: No charge, high penetration.
These forms of radiation differ in their ability to penetrate materials and biological tissues.
Biological Effects of Radiation
Ionizing radiation can remove electrons from atoms, making them more reactive and potentially damaging living cells. The effects depend on the type, energy, and duration of exposure. Rapidly dividing cells are most susceptible to damage, which can lead to cancer or radiation sickness.
Radiation Type | Travel Distance in Air | Tissue Penetration | Protective Shielding |
|---|---|---|---|
Alpha (α) | A few cm | Stops at skin surface | Paper, clothing |
Beta (β) | A few meters | Will not penetrate past skin layer | Heavy clothing, plastic, aluminum foil, gloves |
Gamma (γ) | Several hundred meters | Fully penetrates body | Thick lead, concrete, water |
X-ray | Several meters | Penetrates tissues but not bone | Lead apron, concrete barrier |
Neutron (n) | Hundreds to thousands of meters | Fully penetrates body | Concrete barrier, water |
Nuclear Equations and Radioactive Decay
Writing Nuclear Equations
Nuclear equations represent the transformation of one nucleus into another during radioactive decay. The sum of atomic numbers and mass numbers must be equal on both sides of the equation.
Alpha Decay:
Beta Decay:
Gamma Decay:
Positron Emission:
Example: Americium-241 alpha decay:
Radiation Units and Half-Lives
Measuring Radioactivity
The activity of a radioactive sample is measured in disintegrations per second. Common units include:
Becquerel (Bq): 1 disintegration per second (SI unit).
Curie (Ci): 3.7 × 1010 disintegrations per second.
Half-Life Calculations
The half-life of a radioisotope is the time required for half of the radioactive atoms in a sample to decay. The amount remaining after n half-lives is given by:
Example: If the half-life of iodine-131 is 8.0 days, after 32 days (4 half-lives), a 100 mg sample will have mg remaining.
Medical Applications for Radioisotopes
Diagnostic and Therapeutic Uses
Radioisotopes are widely used in medicine for both diagnosis and treatment. Diagnostic procedures use small amounts of radioisotopes (tracers) with short half-lives to minimize radiation exposure. Therapeutic applications use higher doses to destroy diseased tissues, such as cancer cells.
Imaging and Cancer Treatment
Technetium-99m is commonly used for imaging blood flow in the lungs. Areas with abnormal tracer distribution appear as "cold" or "hot" spots, indicating nonfunctioning or hyperactive tissue, respectively.
In cancer therapy, radioisotopes such as iodine-131 or cobalt-60 are used to target and destroy cancerous tissues. Brachytherapy involves implanting small radioactive seeds directly into tumors.
Positron Emission Tomography (PET)
PET scans use radioisotopes like fluorine-18 to detect functional abnormalities in organs, especially the brain. The emitted positrons interact with electrons, producing gamma rays that are detected to create detailed images of tissue activity.
