Radioactivity, Nuclear Chemistry, and Biomolecules: Study Notes for Introductory Chemistry

Radioactivity and Nuclear Chemistry

Introduction to Radioactivity

Radioactivity is the spontaneous emission of particles or energy from an unstable atomic nucleus. This process, known as radioactive decay, allows unstable nuclei to become more stable by releasing energy or subatomic particles.

• Radioactive decay: The process by which an unstable nucleus loses energy by emitting radiation.

• Unstable nuclei: Atoms with an imbalance in the number of protons and neutrons, or with too large a nucleus, tend to undergo radioactive decay.

Discovery of Radioactivity

• Antoine Becquerel (1896): Discovered that uranium emits radiation even in the absence of light.

• Marie Curie: Discovered the elements polonium and radium, named the phenomenon "radioactivity," and won two Nobel Prizes for her work.

Why Atoms Become Radioactive

• Atoms become unstable due to:

  • Too many protons

  • Too many neutrons

  • Incorrect neutron-to-proton ratio

  • Large nuclei require more neutrons than small nuclei to remain stable

Types of Radiation

There are four main types of radiation emitted during radioactive decay:

Radiation	Symbol	What is Emitted?	Charge	Mass
Alpha	\( \alpha \)	2 protons + 2 neutrons	+2	4
Beta	\( \beta^- \)	Electron	-1	0
Gamma	\( \gamma \)	Energy (photon)	0	0
Positron	\( \beta^+ \)	Positron	+1	0

Nuclear Equations

Nuclear equations represent the changes that occur during radioactive decay. Unlike chemical reactions, nuclear reactions can change one element into another by altering the number of protons in the nucleus.

• Mass number (A): The total number of protons and neutrons; must balance on both sides of the equation.

• Atomic number (Z): The number of protons; must also balance on both sides.

Alpha Decay (\( \alpha \))

• An alpha particle (\( ^4_2\mathrm{He} \)) contains 2 protons and 2 neutrons.

• When an atom emits an alpha particle:

  • Mass number decreases by 4

  • Atomic number decreases by 2

• Example:

$^{238}_{92}\mathrm{U} \rightarrow ^{234}_{90}\mathrm{Th} + ^4_2\mathrm{He}$

Beta Decay (\( \beta^- \))

• A beta particle (\( ^0_{-1}\mathrm{e} \)) is an electron emitted from the nucleus.

• During beta decay, a neutron changes into a proton.

• Mass number remains the same; atomic number increases by 1.

• Example:

$^{228}_{88}\mathrm{Ra} \rightarrow ^{228}_{89}\mathrm{Ac} + ^0_{-1}\mathrm{e}$

Positron Emission (\( \beta^+ \))

• A positron (\( ^0_{+1}\mathrm{e} \)) is a positively charged electron.

• During positron emission, a proton changes into a neutron.

• Mass number remains the same; atomic number decreases by 1.

• Example:

$^{30}_{15}\mathrm{P} \rightarrow ^{30}_{14}\mathrm{Si} + ^0_{+1}\mathrm{e}$

Gamma Radiation (\( \gamma \))

• Gamma rays (\( ^0_0\gamma \)) are high-energy photons (energy, not matter).

• Gamma emission does not change the mass number or atomic number.

• Often emitted alongside alpha or beta decay.

Radiation	Mass Number Change	Atomic Number Change
Alpha (\( \alpha \))	-4	-2
Beta (\( \beta^- \))	0	+1
Positron (\( \beta^+ \))	0	-1
Gamma (\( \gamma \))	0	0

Ionizing Power vs. Penetrating Power

• Ionizing Power: The ability of radiation to remove electrons from atoms, creating ions. Higher ionizing power means more potential damage to cells.

• Penetrating Power: The ability of radiation to pass through matter. Higher penetrating power means the radiation can travel farther through materials and tissue.

Half-Life

The half-life of a radioactive isotope is the time required for half of a sample to decay into daughter nuclides. The amount of radioactive material never reaches zero; it continues to halve over each successive half-life.

Nuclear Fission vs. Nuclear Fusion

• Nuclear Fission: The splitting of a large nucleus (such as uranium-235) into two smaller nuclei, releasing energy and additional neutrons. These neutrons can initiate further fission reactions (chain reaction).

• Nuclear Fusion: The combining of two small nuclei (such as hydrogen) to form a larger nucleus (such as helium), releasing energy. Fusion powers the sun and stars.

• Example (Fusion):

$\text{Hydrogen} + \text{Hydrogen} \rightarrow \text{Helium} + \text{Energy}$

Effects of Radiation and Medical Uses

• Radiation can damage cells by ionizing molecules, especially DNA.

• The effects depend on the dose and duration of exposure.

Acute Radiation Exposure

• Large dose over a short period

• Possible effects: weakened immune system, intestinal damage, infection, death (at very high doses)

Long-Term Radiation Exposure

• Smaller doses over a long period

• Can damage DNA, increase cancer risk, and cause abnormal cell growth

Measuring Radiation

• Rem (roentgen equivalent man): A unit that measures human exposure to radiation, accounting for the type and ionizing power of the radiation.

Sources of Radiation

• Most exposure comes from natural background sources, especially radon gas (a product of uranium decay).

Medical Uses of Radioactivity

• Diagnosis:

  • Technetium-99: bone scans

  • Phosphorus-32: imaging tumors

  • Iodine-131: diagnosing thyroid disorders

• Treatment: Gamma rays from radioisotopes (e.g., cobalt-60) are used to treat cancer by targeting and destroying tumor cells.

Biomolecules: Structure and Function

The Four Major Biomolecules

Biomolecule	Main Function
Carbohydrates	Short-term energy
Lipids	Long-term energy & cell membranes
Proteins	Structure, enzymes, body functions
Nucleic Acids	Store genetic information (DNA & RNA)

Carbohydrates

• Main source of short-term energy

• Major structural component of plants

Types of Carbohydrates

• Monosaccharides: Single sugar molecules (e.g., glucose, fructose, galactose). Cannot be broken down into smaller carbohydrates.

• Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose = glucose + fructose, lactose).

• Polysaccharides: Many monosaccharides joined together; complex carbohydrates (e.g., starch, glycogen, cellulose).

Type	Examples	Main Function
Monosaccharide	Glucose, Fructose, Galactose	Immediate energy
Disaccharide	Sucrose, Lactose	Two sugars joined
Polysaccharide	Starch, Glycogen, Cellulose	Energy storage or structure

• Starch: Plant energy storage; digestible by humans (found in potatoes, rice, bread, pasta).

• Glycogen: Animal storage form of glucose; stored in liver and muscles.

• Cellulose: Main structural component of plants; indigestible by humans (dietary fiber).

Lipids

• Insoluble in water; soluble in nonpolar solvents

• Store long-term energy, insulate the body, and form cell membranes

Types of Lipids

• Fatty acids

• Triglycerides (fats & oils)

• Phospholipids

• Glycolipids

• Steroids

Fatty Acids

• Carboxylic acids with long hydrocarbon chains ("tails")

• The hydrocarbon tail makes them insoluble in water

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids: No carbon-carbon double bonds; usually solid at room temperature (e.g., butter, beef fat, coconut oil).

• Unsaturated fatty acids: One or more carbon-carbon double bonds; usually liquid at room temperature (e.g., olive oil, peanut oil, corn oil).

Triglycerides (Fats & Oils)

• Composed of one glycerol and three fatty acids joined by ester linkages.

• Saturated fats: Made from saturated fatty acids; solid at room temperature.

• Unsaturated fats (oils): Made from unsaturated fatty acids; liquid at room temperature.

Phospholipids

• Major component of cell membranes.

• Structure: Polar (hydrophilic) head and nonpolar (hydrophobic) tails.

Cell Membrane Structure

• Composed of a lipid bilayer: polar heads face outward toward water, nonpolar tails face inward.

Steroids

• Lipids with a four-ring structure (e.g., cholesterol, testosterone, estrogen).

• Cholesterol: Part of cell membranes and precursor for steroid hormones.

Dietary Fats

• High intake of saturated fats increases risk of artery blockages, heart attacks, and strokes.

• Monounsaturated fats may help protect against these conditions.

Proteins and Amino Acids

• Proteins are polymers of amino acids and perform essential functions:

  • Catalyze chemical reactions (enzymes)

  • Form muscles, skin, cartilage

  • Transport oxygen

  • Fight disease (antibodies)

  • Act as hormones

Enzymes

• Proteins that act as biological catalysts, speeding up chemical reactions necessary for life.

Amino Acids

• Building blocks of proteins; each contains:

  • Amine group (–NH2)

  • Carboxylic acid group (–COOH)

  • R group (side chain) that varies among amino acids

Peptide Bonds and Protein Chains

• Peptide bond: Forms when the amine group of one amino acid reacts with the carboxylic acid group of another, joining amino acids into a chain.

• Dipeptide: Two amino acids joined

• Polypeptide: Many amino acids joined

• Protein: One or more folded polypeptide chains

Protein Structure

• Primary structure: Sequence of amino acids (held by peptide bonds)

• Secondary structure: Local folding patterns (alpha helix, beta pleated sheet) stabilized by hydrogen bonds

• Tertiary structure: Overall 3D shape of a single chain, maintained by interactions between R groups (hydrogen bonds, disulfide linkages, hydrophobic interactions, salt bridges)

• Quaternary structure: Two or more polypeptide chains combine to form a functional protein (e.g., hemoglobin)

Nucleic Acids: DNA and RNA

• Nucleic acids store and transmit genetic information.

• Two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded	Usually single-stranded
Location	Mainly in nucleus	Nucleus & cytoplasm

• Nucleotides: Building blocks of DNA and RNA, each containing a phosphate group, a sugar, and a nitrogenous base.

• DNA bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

• RNA bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

Genetic Information

• Codon: Sequence of three bases that codes for one amino acid.

• Gene: Sequence of codons containing instructions for making one protein.

• Chromosomes: Long DNA molecules containing many genes; humans have 46 chromosomes.

DNA Replication

• Process of making an exact copy of DNA before cell division.

• Steps:

  1. DNA double helix unwinds

  2. Hydrogen bonds between bases break

  3. Each original strand serves as a template

  4. Complementary bases are added

  5. Two identical DNA molecules are produced

Protein Synthesis

• How the body makes proteins from genetic instructions.

• Step 1: Transcription: A gene in DNA is copied into messenger RNA (mRNA), which leaves the nucleus for the ribosome.

• Step 2: Translation: At the ribosome, mRNA is read one codon at a time; the correct amino acids are brought in, peptide bonds form, and a protein is synthesized.