Atomic Structure and Radioisotopes: Study Notes

Atomic Structure and Radioisotopes

Overview

This chapter introduces the fundamental concepts of atomic structure, the periodic table, isotopes, and the nature and effects of radioactivity. Understanding these topics is essential for further study in chemistry and related sciences.

Elements and the Structure of the Atom

Classification of Matter

• Matter can be classified as either a pure substance or a mixture.

• Pure substances include elements (composed of one type of atom) and compounds (composed of two or more types of atoms chemically bonded).

• Mixtures can be homogeneous (uniform composition, e.g., salt water) or heterogeneous (non-uniform composition, e.g., salad).

Structure of the Atom

• An atom consists of a dense, positively charged nucleus containing protons and neutrons, surrounded by a cloud of electrons.

• The nucleus is extremely small compared to the overall size of the atom.

• Electrons occupy regions called orbitals outside the nucleus.

Subatomic Particles

Particle	Symbol	Charge	Mass (g)	Mass (amu)	Location
Proton	p or p+	1+	1.67 × 10−24	1.007	Nucleus
Neutron	n or n0	0	1.67 × 10−24	1.008	Nucleus
Electron	e−	1−	9.11 × 10−28	0.00055	Electron orbitals

Atomic Number and Mass Number

• Atomic number (Z): Number of protons in the nucleus; defines the element.

• Mass number (A): Total number of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element (same Z) with different numbers of neutrons (different A).

Symbol Notation for Isotopes

• Isotopes are represented as: $^A_ZX$ where X is the element symbol, A is the mass number, and Z is the atomic number.

• Example: $^{14}_6C$ represents carbon-14.

Example Table: Naturally Occurring Isotopes of Carbon

Isotope	Atomic Number	Number of Protons	Number of Neutrons
Carbon-12	6	6	6
Carbon-13	6	6	7
Carbon-14	6	6	8

Each isotope has its own unique percent natural abundance.

The Periodic Table of the Elements

Organization

• Elements are arranged in periods (horizontal rows) and groups (vertical columns) by increasing atomic number.

• Main group elements are in Groups 1A–8A; transition elements are in Groups 3B–12B.

• Elements in the same group share similar chemical and physical properties.

Classification of Elements

• Metals: Shiny, malleable, ductile, good conductors of heat and electricity; mostly solids at room temperature.

• Nonmetals: Poor conductors, good insulators; can be solids, liquids, or gases at room temperature.

• Metalloids: Have properties intermediate between metals and nonmetals.

Electron Arrangement and Valence Electrons

Quantum Mechanical Model

• Electrons occupy specific energy levels and sublevels (shells and subshells) around the nucleus.

• Each shell is designated by a principal quantum number (n = 1, 2, 3, ...).

• Subshells are labeled s, p, d, f, etc.

Shells and Subshells

Shell (n)	Number of Subshells	Subshell Letters
1	1	s
2	2	s, p
3	3	s, p, d
4	4	s, p, d, f

• Valence electrons: Electrons in the outermost shell; determine chemical reactivity.

Isotopes and Radioactivity

Radioisotopes

• Radioisotopes are unstable isotopes that undergo radioactive decay to become more stable.

• Decay can emit particles (alpha, beta) or electromagnetic radiation (gamma rays).

Types of Radioactive Decay

• Alpha (α) decay: Emission of an alpha particle ($^4_2He$ nucleus, 2 protons and 2 neutrons, +2 charge).

• Beta (β) decay: Emission of a high-energy electron ($e^-$), resulting from the conversion of a neutron to a proton.

• Gamma (γ) decay: Emission of high-energy electromagnetic radiation; often accompanies other types of decay.

Example: Alpha Decay Equation

• General form: $^A_ZX \rightarrow ^{A-4}_{Z-2}Y + ^4_2He$

• Example: $^{226}_{88}Ra \rightarrow ^{222}_{86}Rn + ^4_2He$

Half-Life

• Half-life (t1/2): The time required for half of the radioactive nuclei in a sample to decay.

• Fraction remaining after n half-lives: $N = (\frac{1}{2})^n$

Table: Common Radioisotopes and Their Decay Types

Radioisotope	Type of Decay
Barium-131	β
Carbon-14	β
Chromium-51	γ
Gallium-67	γ
Gold-198	β
Iodine-131	β and γ
Iron-59	β and γ
Krypton-85	β
Phosphorus-32	β
Technetium-99m	γ
Uranium-238	α

Additional info: Table entries inferred from context and standard radioisotope decay types.

Electromagnetic Radiation

Nature of Electromagnetic Radiation

• Electromagnetic radiation (light) is energy that travels through space as waves.

• Characterized by wavelength (λ), frequency (ν), and energy (E).

• Relationship: $E = h\nu = \frac{hc}{\lambda}$ where h = Planck's constant, c = speed of light.

Penetrating Power and Biological Effects of Radiation

Penetrating Power

• Alpha particles: Low penetrating power; stopped by paper.

• Beta particles: Moderate penetrating power; stopped by thin metal (e.g., aluminum).

• Gamma rays: High penetrating power; require thick lead or concrete for shielding.

Biological Effects

• Ionizing radiation (alpha, beta, gamma, X-rays, high-energy UV) can remove electrons from atoms, causing damage to biological tissues.

• Effects include DNA mutations, cell death, and increased cancer risk.

• Rapidly dividing cells are most sensitive to radiation damage.

Measuring Radiation Exposure

• Gray (Gy): Measures absorbed dose (energy per unit mass).

• Sievert (Sv): Measures biological effect, accounting for type and energy of radiation.

• Average background radiation: ~2–4 mSv/year.

Summary Table: Penetrating Power of Radiation

Type of Radiation	Penetrating Power	Shielding Material
Alpha (α)	Low	Paper
Beta (β)	Moderate	Aluminum
Gamma (γ)	High	Lead, concrete

Key Objectives

• Describe the structure of atoms and the properties of subatomic particles.

• Use the periodic table to classify elements and predict properties.

• Determine atomic number, mass number, and isotope notation for elements.

• Identify types of radioactive decay and write nuclear equations.

• Explain the biological effects and measurement of nuclear radiation.