Atoms, Atomic Structure, Isotopes, and Radioactivity – Chapter 2 Study Notes

Atoms and Their Components

Definition and Structure of Atoms

An atom is the fundamental unit of matter, composed of a particular type of element. All substances are made up of atoms, which determine their chemical properties.

• Elements are pure substances composed of only one type of atom.

• Examples: Diamond (pure carbon), Graphite (carbon atoms in sheets), and the lead in a pencil (also carbon in a different form).

Subatomic Particles

Types and Properties of Subatomic Particles

Atoms are made up of three main subatomic particles: protons, neutrons, and electrons. Each has distinct properties and locations within the atom.

• Proton (p or p+): Positively charged, found in the nucleus.

• Neutron (n or n0): Uncharged (neutral), found in the nucleus.

• Electron (e-): Negatively charged, orbits the nucleus.

Subatomic Particle	Symbol	Electrical Charge	Relative Mass
Electron	e-	1–	0.0005 (1/2000)
Proton	p or p+	1+	1
Neutron	n or n0	0	1

Atomic Mass Unit (amu)

Definition and Application

The atomic mass unit (amu) is a standard unit of mass used to express atomic and molecular weights. It is defined as one-twelfth the mass of a carbon-12 atom.

• Both a proton and a neutron have a mass of approximately 1 amu.

• Electrons have a much smaller mass, about 1/2000 that of a proton or neutron.

• Example: Carbon atom (6 protons, 6 neutrons) has a mass close to 12 amu.

Atomic Number and Mass Number

Identifying Elements and Calculating Neutrons

Each element is defined by its atomic number (number of protons). The mass number is the sum of protons and neutrons in the nucleus.

• Atomic Number (Z): Number of protons, unique to each element.

• Mass Number (A): Total number of protons and neutrons.

• For a neutral atom, number of protons = number of electrons.

• Number of neutrons = Mass number – Atomic number.

Example notation: $^{12}_{6} ext{C}$ (Carbon-12: 6 protons, 6 neutrons)

Isotopes and Atomic Mass

Definition and Examples of Isotopes

Isotopes are atoms of the same element that have different numbers of neutrons, resulting in different mass numbers. All isotopes of an element have the same number of protons and electrons.

• Example: Carbon has three common isotopes:

  • Carbon-12: $^{12}_{6} ext{C}$ (6 protons, 6 neutrons)

  • Carbon-13: $^{13}_{6} ext{C}$ (6 protons, 7 neutrons)

  • Carbon-14: $^{14}_{6} ext{C}$ (6 protons, 8 neutrons)

• Isotopes are important in chemical, biological, and medical applications.

Practice: Determining Subatomic Particles

Using the Periodic Table

The periodic table provides atomic numbers and atomic masses for all elements, which can be used to determine the number of protons, neutrons, and electrons in an atom.

• Atomic number (top number) = number of protons.

• Mass number (rounded atomic mass) – atomic number = number of neutrons.

• For neutral atoms, number of electrons = number of protons.

Radioactivity and Isotopes

Radioactive Decay and Types of Radiation

Radioactivity is the spontaneous emission of energy from the nucleus of unstable isotopes. Some isotopes are naturally radioactive, while others are man-made.

• Radioactive decay releases energy in the form of particles or electromagnetic waves.

• Common types of radiation:

  • Alpha (α) particles: Helium nuclei ($^{4}_{2} ext{He}$), low penetration, stopped by paper.

  • Beta (β) particles: High-energy electrons, moderate penetration, stopped by aluminum.

  • Gamma (γ) rays: High-energy electromagnetic radiation, high penetration, requires lead or concrete for shielding.

  • Positron (β+) emission: Emission of a positively charged electron.

  • Neutron emission: Release of neutrons from the nucleus.

Type of Radiation	Symbol	Penetration	Shielding Material
Alpha	α or $^{4}_{2} ext{He}$	Low	Paper, skin
Beta	β	Moderate	Aluminum
Gamma	γ	High	Lead, concrete

Measuring Radioactivity

Units and Biological Effects

Radioactivity is measured by the number of disintegrations per second. The SI unit is the Becquerel (Bq), and the Curie (Ci) is a commonly used unit in medicine.

• 1 Curie (Ci) = $3.7 imes 10^{10}$ disintegrations per second.

• Biological effects depend on the type and amount of radiation absorbed.

• Radiation can damage living tissue, leading to radiation sickness or therapeutic effects.

Half-Life and Medical Applications

Half-Life of Radioisotopes

The half-life of a radioactive isotope is the time required for half of the atoms in a sample to decay. This property is crucial for medical and scientific applications.

• Physical half-life: Time for half the atoms to decay.

• Biological half-life: Time for half the substance to be eliminated from the body.

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

Medical Uses of Radioisotopes

Radioisotopes are widely used in medicine for diagnosis and treatment.

• Diagnosis: Radioisotopes can be used to trace the rate of uptake in organs or tissues (e.g., PET scans).

• Treatment: Radioisotopes are administered to destroy diseased or cancerous cells, often targeting rapidly dividing cells.

• External radiation therapy uses focused beams; internal therapy involves implanting or injecting radioisotopes.

Nuclear Equations and Radioactive Decay

Writing Nuclear Equations

Nuclear equations represent the transformation of a nucleus during radioactive decay, showing the new nucleus and emitted radiation.

• General form: $\text{Parent nucleus} \rightarrow \text{Daughter nucleus} + \text{Radiation}$

• Example (Alpha decay): $^{222}_{86}\text{Rn} \rightarrow ^{218}_{84}\text{Po} + ^{4}_{2}\text{He}$

• Example (Beta decay): $^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + \beta^{-}$

Positron Emission and PET Scans

Positron emission is used in Positron Emission Tomography (PET) scans to detect metabolic activity in tissues.

• Positron ($\beta^{+}$) emitted interacts with an electron, producing gamma rays detected by the scanner.

Additional info: Some content was inferred and expanded for completeness, including definitions, examples, and standard nuclear equations.