Atoms and Radioactivity: Structured Study Notes for General, Organic, and Biological Chemistry

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 basic chemistry concepts.

• Protons: Positively charged particles located in the nucleus.

• Neutrons: Neutral particles also found in the nucleus.

• Electrons: Negatively charged particles dispersed in the electron cloud surrounding the nucleus.

• Atomic neutrality: Atoms are electrically neutral because the number of protons equals the number of electrons.

• Atomic mass unit (amu): A unit used to express atomic and subatomic masses; 1 amu is defined as one-twelfth the mass of a carbon-12 atom.

• Mass distribution: Most of the atom's mass is concentrated in the nucleus; electrons contribute negligibly to mass.

2.2 Atomic Number and Mass Number

The atomic number and mass number are key identifiers for elements and their isotopes. These numbers help determine the composition and properties of atoms.

• Atomic Number (Z): The number of protons in an atom; unique for each element and found above the element symbol in the periodic table.

• Mass Number (A): The sum of protons and neutrons in an atom. $\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$

• Symbolic Notation: Atoms can be represented as $^A_Z\text{Element}$, where A is the mass number and Z is the atomic number.

• Determining Subatomic Particles:

  1. Find the element on the periodic table.

  2. Use atomic number for protons and electrons.

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

  4. Write symbolic notation.

2.3 Isotopes and Atomic Mass

Isotopes are variants of elements with the same number of protons but different numbers of neutrons. Atomic mass reflects the weighted average of all naturally occurring isotopes.

• Isotopes: Atoms of the same element with different mass numbers due to varying neutron counts.

• Notation: Isotopes can be written as symbolic notation or with the element name and mass number (e.g., carbon-12).

• Atomic Mass: The average mass of all isotopes, weighted by their natural abundance.

2.4 Radioactivity and Radioisotopes

Radioactivity is the spontaneous emission of energy from unstable atomic nuclei. Radioisotopes are isotopes that emit radiation as they decay to become stable.

• Radioactivity: Energy emitted from the nucleus; discovered by Henri Becquerel.

• Radioisotopes: Isotopes that undergo radioactive decay, emitting radiation.

• Forms of Radiation:

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

  • Beta particles (β): High-energy electrons, 1- charge; emitted when a neutron becomes a proton.

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

  • Positrons (β+): Positive charge, same mass as beta particles; emitted when a proton becomes a neutron.

  • Neutrons: No charge; can induce radioactive decay when added to nuclei.

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

• Radiation Units:

  • Sievert (Sv): SI unit for biological damage.

  • Millisievert (mSv): Commonly used for measuring exposure.

  • Curie (Ci): Unit for radioactivity.

  • Becquerel (Bq): SI unit for disintegrations per second.

• Radiation Sickness: Depends on dose and exposure time; rapidly dividing cells are most affected.

2.5 Nuclear Equations and Radioactive Decay

Nuclear equations describe the transformation of elements during radioactive decay. Balancing these equations requires equal atomic and mass numbers on both sides.

• Writing Nuclear Equations:

  1. Write symbolic notation for the decaying radioisotope.

  2. Place emitted radiation on the product side.

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

• Types of Decay:

  • Alpha decay: Loss of an alpha particle.

  • Beta decay: Emission of a beta particle; neutron becomes a proton.

  • Positron emission: Proton becomes a neutron; positron emitted.

  • Gamma decay: Emission of gamma rays; no change in atomic or mass number.

• Production of Radioisotopes: Bombarding stable isotopes with particles (alpha, protons, neutrons) creates radioisotopes.

2.6 Radiation Units and Half-Lives

Radioactivity is measured in units such as curie and becquerel. The half-life is the time required for half of a radioactive sample to decay, important for medical and environmental applications.

• Radioactivity Units:

  • Curie (Ci): Traditional unit for radioactivity.

  • Becquerel (Bq): SI unit for disintegrations per second.

• Half-Life:

  • Physical half-life: Time for 50% decay of atoms in a sample.

  • Biological half-life: Time for elimination from the body.

  • Effective half-life: Combination of physical and biological half-lives.

  • Equation: $\text{Remaining Amount} = \text{Initial Amount} \times \left(\frac{1}{2}\right)^n$ where $n$ is the number of half-lives.

• Medical Isotopes: Short half-lives for rapid elimination; natural radioisotopes have longer half-lives.

2.7 Medical Applications for Radioisotopes

Radioisotopes are used in medicine for both diagnosis and treatment. Their ability to concentrate in specific tissues allows for imaging and targeted therapy.

• Diagnostic Uses:

  • Tracers (e.g., technetium-99m) are used in imaging to detect blood flow and tissue function.

  • Cold spots indicate nonfunctioning areas; hot spots indicate high activity (e.g., cancer).

• Therapeutic Uses:

  • Radioisotopes (e.g., iodine-131) destroy diseased tissues, especially rapidly dividing cells.

  • External beam radiation (gamma rays from cobalt-60) and brachytherapy (radioactive seeds) are used for cancer treatment.

• PET Scans:

  • Positron emission tomography uses fluorine-18 to detect functional abnormalities, especially in the brain.

  • Positron emission produces gamma rays detected by scanners.

Summary Table: Types of Ionizing Radiation

Type	Symbol	Charge	Penetrating Power	Biological Effect
Alpha particle	α	+2	Low	High if ingested
Beta particle	β	-1	Moderate	Moderate
Gamma ray	γ	0	High	High
Positron	β+	+1	Moderate	Moderate
Neutron	n	0	Variable	Variable

Example: Writing a Nuclear Decay Equation

• Uranium-238 undergoes alpha decay: $^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\text{He}$

• Beta decay example: $^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^{0}_{-1}\text{e}$

Example: Calculating Remaining Radioisotope

• If 100 mg of a radioisotope with a half-life of 3 hours is left for 9 hours, the remaining amount is: $100 \times \left(\frac{1}{2}\right)^3 = 12.5$ mg

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