Chapter 2 Study Guide: Atoms, Elements, Chemical Bonds, and Biomolecules

2.1 Atoms and Elements

Major Elements and Atomic Structure

This section introduces the fundamental building blocks of matter—atoms and elements—and explores their structure and properties.

• Major Elements: The most abundant elements in the human body are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N).

• Subatomic Particles:

  • Proton: Positively charged particle found in the nucleus; determines the atomic number and identity of the element.

  • Neutron: Neutral particle in the nucleus; contributes to atomic mass and isotope formation.

  • Electron: Negatively charged particle orbiting the nucleus; involved in chemical bonding and reactions.

• Electron Shells: Electrons occupy energy levels (shells) around the nucleus. The first shell holds up to 2 electrons, the second up to 8, and so on. The outermost shell (valence shell) is most important for chemical bonding.

• Atomic Number and Mass Number:

  • Atomic Number (Z): Number of protons in the nucleus.

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

• Isotopes: Atoms of the same element with different numbers of neutrons. Some isotopes are radioactive and decay over time, emitting radiation.

Example: Carbon-12 and Carbon-14 are isotopes of carbon; Carbon-14 is radioactive and used in radiocarbon dating.

2.2 Matter Combined: Mixtures and Chemical Bonds

Ions, Molecules, and Compounds

This section explains how atoms combine to form ions, molecules, and compounds, and the types of chemical bonds involved.

• Ion: An atom or molecule with a net electric charge due to the loss or gain of electrons. Cation is positively charged; anion is negatively charged.

• Molecule: Two or more atoms covalently bonded together (e.g., O2).

• Compound: Substance formed from two or more different elements chemically bonded (e.g., H2O).

• Types of Chemical Bonds:

  • Ionic Bond: Transfer of electrons from one atom to another, forming ions (e.g., NaCl in the body).

  • Covalent Bond: Sharing of electron pairs between atoms.

    • Nonpolar Covalent Bond: Electrons shared equally.

    • Polar Covalent Bond: Electrons shared unequally, creating partial charges (e.g., H2O).

  • Hydrogen Bond: Weak attraction between a hydrogen atom (already covalently bonded to an electronegative atom) and another electronegative atom. Important in water, DNA, and proteins.

• Role of Hydrogen Bonds: Stabilize protein and nucleic acid structures; contribute to water's unique properties (cohesion, high specific heat).

• Surface Tension: Caused by hydrogen bonding between water molecules; important in respiratory physiology for alveolar function.

Example: Hydrogen bonds hold the two strands of DNA together.

2.3 Chemical Reactions

Types and Properties of Chemical Reactions

Chemical reactions involve the making and breaking of chemical bonds, resulting in new substances.

• Reactants and Products: Reactants are starting substances; products are formed as a result of the reaction.

• Types of Chemical Reactions:

  • Synthesis (Anabolism): Two or more substances combine to form a more complex product. General equation: $A + B \rightarrow AB$

  • Decomposition (Catabolism): A complex molecule breaks down into simpler substances. General equation: $AB \rightarrow A + B$

  • Exchange: Parts of molecules are exchanged to form new compounds. General equation: $AB + CD \rightarrow AD + CB$

• Energy in Reactions: Chemical reactions involve energy changes. Activation energy is the minimum energy required to start a reaction.

• Forms of Energy: Chemical (stored in bonds), electrical (movement of charged particles), and potential energy (stored energy).

• Exergonic vs. Endergonic Reactions:

  • Exergonic: Release energy (e.g., cellular respiration).

  • Endergonic: Require energy input (e.g., protein synthesis).

• Catalysts: Substances that speed up reactions without being consumed (e.g., enzymes).

Example: The breakdown of glucose in cellular respiration is an exergonic reaction.

2.4 Inorganic Compounds: Water, Acids, Bases, and Salts

Properties and Functions of Inorganic Compounds

Inorganic compounds are essential for life and include water, acids, bases, and salts.

• Inorganic vs. Organic Compounds: Inorganic compounds generally do not contain carbon-hydrogen bonds (e.g., water, salts); organic compounds do (e.g., glucose).

• Bonds in Water: Water molecules are held together by polar covalent bonds; hydrogen bonds form between molecules.

• Functions of Water:

  • Solvent for chemical reactions

  • Temperature regulation

  • Lubrication and cushioning

  • Transport of substances

• Key Terminology:

  • Solution: Homogeneous mixture of solute and solvent.

  • Solvent: Substance that dissolves another (water is the universal solvent).

  • Solute: Substance dissolved in a solvent.

  • Hydrophilic: Water-attracting.

  • Hydrophobic: Water-repelling.

• Acids, Bases, and Salts:

  • Acid: Releases H+ ions in solution (proton donor).

  • Base: Accepts H+ ions or releases OH- (proton acceptor).

  • Salt: Ionic compound formed from acid-base reaction.

• pH Scale: Measures hydrogen ion concentration; ranges from 0 (acidic) to 14 (basic), with 7 as neutral. $\text{pH} = -\log_{10}[\text{H}^+]$

• Buffer System: Maintains pH stability in the body (e.g., bicarbonate buffer in blood).

• Electrolytes: Salts that dissociate in water to form ions; important for nerve and muscle function.

Example: Table salt (NaCl) dissociates into Na+ and Cl- ions in water.

2.5 Organic Compounds: Carbohydrates, Lipids, Proteins, and Nucleic Acids

Structure and Function of Biomolecules

Organic compounds are carbon-based molecules essential for life. The four major classes are carbohydrates, lipids, proteins, and nucleic acids.

• Dehydration Synthesis: Chemical reaction that joins monomers by removing water.

• Hydrolysis: Chemical reaction that breaks polymers into monomers by adding water.

• Biomolecule Table:

Biomolecule	Monomer	Functions	Hydrophilic/Hydrophobic
Carbohydrates	Monosaccharides (e.g., glucose)	Energy source, structural support	Hydrophilic
Lipids	Fatty acids, glycerol	Energy storage, insulation, cell membranes	Hydrophobic
Proteins	Amino acids	Enzymes, structure, transport, signaling	Varies (mostly hydrophilic)
Nucleic Acids	Nucleotides	Genetic information storage and transfer	Hydrophilic

• ATP (Adenosine Triphosphate): The primary energy carrier in cells. Hydrolysis of ATP releases energy for cellular processes. $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Example: Enzymes are proteins that catalyze biochemical reactions; DNA is a nucleic acid storing genetic information.