chapter 2 : Atoms and Radioactivity

Atoms and Radioactivity

Introduction

This section covers the fundamental concepts of atomic structure, isotopes, radioactivity, and their applications, as outlined in a typical GOB Chemistry curriculum. Understanding these topics is essential for grasping the behavior of matter and the role of nuclear processes in chemistry and medicine.

Atoms

Definition and Structure

• Atom: The smallest unit of an element that retains the chemical properties of that element.

• Composed of a nucleus (containing protons and neutrons) and electrons in surrounding energy levels.

• Atoms are mostly empty space, with a dense central nucleus.

Subatomic Particles

• Proton (p+): Positively charged particle in the nucleus; mass ≈ 1 amu.

• Neutron (n0): Neutral particle in the nucleus; mass ≈ 1 amu.

• Electron (e-): Negatively charged particle outside the nucleus; mass ≈ 0.0005 amu.

Structure of the Atom

• Nucleus contains protons and neutrons.

• Electrons occupy regions called energy levels or shells around the nucleus.

• Atoms are electrically neutral when the number of protons equals the number of electrons.

Atomic Number and Mass Number

Atomic Number (Z)

• Number of protons in the nucleus of an atom.

• Defines the element (e.g., all hydrogen atoms have Z = 1).

• Symbolized as Z.

Mass Number (A)

• Total number of protons and neutrons in the nucleus.

• Symbolized as A.

• Formula: $A = Z + N$ where $N$ is the number of neutrons.

Isotopes

Definition and Notation

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

• Isotopic notation: $^{A}_{Z}X$ where X is the element symbol.

• Example: $^{12}_{6}C$, $^{13}_{6}C$, $^{14}_{6}C$ (carbon isotopes).

Atomic Mass and Average Atomic Mass

Atomic Mass

• The weighted average of the masses of all naturally occurring isotopes of an element.

• Measured in atomic mass units (amu).

Average Atomic Mass Calculation

• Calculated using the relative abundance and mass of each isotope: $\text{Average Atomic Mass} = \sum (\text{fractional abundance} \times \text{isotopic mass})$

• Example: If an element has two isotopes, A (mass = 10 amu, 20% abundance) and B (mass = 11 amu, 80% abundance): $\text{Average Atomic Mass} = (0.20 \times 10) + (0.80 \times 11) = 10.8$ amu

Radioactivity and Nuclear Chemistry

Radioisotopes

• Unstable isotopes that emit energy as they decay to more stable forms.

• Common in medical and industrial applications.

Types of Nuclear Radiation

• Alpha (α) particles: Helium nuclei; low penetration, high damage if ingested.

• Beta (β) particles: High-energy electrons; moderate penetration.

• Gamma (γ) rays: High-energy electromagnetic radiation; high penetration, low ionization.

Summary Table: Types of Nuclear Radiation

Type	Symbol	Charge	Penetration
Alpha	α or $^4_2He$	+2	Low
Beta	β or $^0_{-1}e$	-1	Moderate
Gamma	γ	0	High

Biological Effects of Radiation

• Ionizing radiation can damage cells and DNA.

• Effects depend on energy, type, and exposure duration.

• Protection involves shielding, limiting exposure time, and increasing distance from the source.

Radioactive Decay and Nuclear Equations

Radioactive Decay

• Unstable nuclei emit particles or energy to become more stable.

• Types include alpha, beta, and gamma emission.

Balancing Nuclear Equations

• Conservation of mass number and atomic number must be maintained.

• Example: $^{238}_{92}U \rightarrow ^{234}_{90}Th + ^{4}_{2}He$ (alpha decay)

Half-Life

Physical Half-Life

• The time required for half the atoms in a radioactive sample to decay.

• Formula: $N = N_0 \left(\frac{1}{2}\right)^n$ where $N$ is the remaining amount, $N_0$ is the initial amount, and $n$ is the number of half-lives elapsed.

Biological and Effective Half-Life

• Biological half-life: Time for half of a substance to be eliminated from a biological system.

• Effective half-life: Combines physical and biological half-lives to describe overall elimination from the body.

Medical Applications of Radioisotopes

• Used in imaging (e.g., PET scans) and cancer therapy (e.g., brachytherapy).

• Short half-life isotopes are preferred to minimize radiation exposure.

Radiation Units

• Sievert (Sv): Measures biological effect of radiation.

• Gray (Gy): Measures absorbed dose.

• Dosimeters are used to monitor exposure.

Summary

• Atoms are composed of protons, neutrons, and electrons.

• Isotopes differ in neutron number and mass number.

• Radioactivity involves the emission of particles or energy from unstable nuclei.

• Understanding nuclear chemistry is essential for applications in medicine and industry.