Atoms, Isotopes, and Radioactivity: Study Guide for GOB Chemistry

Atoms and Their Components

Subatomic Particles

Atoms are the fundamental building blocks of matter, composed of smaller particles called subatomic particles. Understanding these components is essential for studying chemical properties and reactions.

• Protons: Positively charged particles located in the nucleus of the atom. The number of protons defines the atomic number and the identity of the element.

• Neutrons: Neutral particles also found in the nucleus. Neutrons contribute to the mass of the atom but do not affect its charge.

• Electrons: Negatively charged particles that orbit the nucleus in electron shells. Electrons are involved in chemical bonding and reactions.

Location of Subatomic Particles:

• Protons and neutrons are found in the nucleus (center) of the atom.

• Electrons are found in regions called electron shells or energy levels surrounding the nucleus.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Atomic Number and Mass Number

Definitions and Calculations

The atomic number and mass number are key properties that describe the composition of an atom.

• Atomic Number (Z): The number of protons in the nucleus of an atom. It determines the element's identity.

• Mass Number (A): The total number of protons and neutrons in the nucleus.

Formulas:

• Atomic Number: $Z = \text{number of protons}$

• Mass Number: $A = \text{number of protons} + \text{number of neutrons}$

Determining Subatomic Particles:

• Number of protons = atomic number (Z)

• Number of neutrons = mass number (A) - atomic number (Z)

• Number of electrons = number of protons (for a neutral atom)

Symbolic Notation: Atoms are often represented as $^{A}_{Z}\text{X}$, where X is the element symbol, A is the mass number, and Z is the atomic number.

Example: $^{14}_{6}\text{C}$ represents a carbon atom with 6 protons and 8 neutrons.

Isotopes and Atomic Mass

Isotopes and Mass Calculations

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers.

• Isotope: Atoms of the same element (same number of protons) but different numbers of neutrons.

• Mass Number vs. Atomic Mass:

  • Mass Number (A): Whole number sum of protons and neutrons in a specific isotope.

  • Atomic Mass: Weighted average mass of all naturally occurring isotopes of an element, measured in atomic mass units (amu).

Predicting the Most Common Isotope: The isotope with a mass number closest to the atomic mass (rounded to the nearest whole number) is usually the most abundant.

Example: Chlorine has two main isotopes: $^{35}\text{Cl}$ and $^{37}\text{Cl}$. The atomic mass of chlorine is approximately 35.5 amu, indicating $^{35}\text{Cl}$ is more abundant.

Radioactivity and Radioisotopes

Types and Properties of Ionizing Radiation

Some isotopes are unstable and undergo radioactive decay, emitting radiation. These are called radioisotopes.

• Radioactivity: The spontaneous emission of particles or energy from an unstable atomic nucleus.

• Ionizing Radiation: Radiation with enough energy to remove electrons from atoms, creating ions.

Forms of Ionizing Radiation:

• Alpha (α) particles: Consist of 2 protons and 2 neutrons (helium nucleus). Low penetration; stopped by paper or skin.

• Beta (β) particles: High-energy electrons or positrons. Moderate penetration; stopped by aluminum foil.

• Gamma (γ) rays: High-energy electromagnetic waves. High penetration; requires thick lead or concrete to stop.

Penetrating Power Comparison:

• Alpha < Beta < Gamma

Biological Exposure: Radiation exposure is measured in rem (roentgen equivalent man) or mrem (millirem). High doses can be harmful to living tissue.

Example: Alpha particles are dangerous if ingested but not hazardous outside the body due to low penetration.

Nuclear Equations and Radioactive Decay

Writing and Balancing Nuclear Equations

Nuclear equations represent the changes that occur during radioactive decay. Each type of decay has a characteristic equation.

• Alpha Decay: Emission of an alpha particle ($^{4}_{2}\text{He}$). The atomic number decreases by 2, mass number by 4.

• Beta Decay: Emission of a beta particle ($^{0}_{-1}\text{e}$). The atomic number increases by 1, mass number unchanged.

• Gamma Decay: Emission of gamma rays ($\gamma$). No change in atomic or mass number.

General Nuclear Equation:

• Parent Nucleus → Daughter Nucleus + Emitted Particle(s)

Example: $^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\text{He}$ (alpha decay)

Radiation Units and Half-Lives

Measuring and Calculating Radioactivity

Radioactivity is measured using specific units, and the concept of half-life describes how quickly a radioactive substance decays.

• Units of Radioactivity:

  • Curie (Ci): Measures the rate of radioactive decay.

  • Becquerel (Bq): SI unit; 1 Bq = 1 disintegration per second.

  • Rem (roentgen equivalent man): Measures biological damage.

  • Gray (Gy) and Sievert (Sv): SI units for absorbed dose and biological effect, respectively.

• Half-Life ($t_{1/2}$): The time required for half of the radioactive atoms in a sample to decay.

Half-Life Formula:

• $N = N_0 \left(\frac{1}{2}\right)^{\frac{t}{t_{1/2}}}$

• Where $N$ = remaining amount, $N_0$ = initial amount, $t$ = elapsed time, $t_{1/2}$ = half-life.

Dosing Calculations: Used to determine the remaining activity or safe exposure levels for medical and industrial applications.

Example: If a sample starts with 100 mg and has a half-life of 3 days, after 6 days only 25 mg remain.

Medical Applications for Radioisotopes

Diagnosis and Treatment

Radioisotopes have important uses in medicine, both for diagnosing and treating diseases.

• Diagnostic Uses: Radioisotopes such as technetium-99m are used in imaging to trace biological processes.

• Treatment Uses: Radioisotopes like iodine-131 are used to treat conditions such as hyperthyroidism and certain cancers.

Example: PET scans use positron-emitting isotopes to visualize metabolic activity in tissues.

Summary Table: Types of Ionizing Radiation

Type	Symbol	Charge	Penetration	Shielding
Alpha	$^{4}_{2}\text{He}$	+2	Low	Paper, skin
Beta	$^{0}_{-1}\text{e}$	-1	Moderate	Aluminum foil
Gamma	$\gamma$	0	High	Lead, concrete