Chemistry Comes Alive

Introduction

Chemistry is fundamental to understanding Anatomy & Physiology, as all physiological processes are based on chemical interactions. This chapter explores the nature of mixtures, chemical bonds, and chemical reactions, providing a foundation for later study of biological molecules and cellular processes.

Mixtures

Definition and Types of Mixtures

Mixtures are combinations of two or more substances that are physically blended but not chemically bonded. They can be composed of gases, liquids, or solids in any combination. Mixtures are classified based on the uniformity and particle size of their components.

• Homogeneous mixtures (solutions): Uniform composition throughout; no visible separation between components; transparent.

• Heterogeneous mixtures: Non-uniform composition; visible differences between components; can be translucent or opaque.

Types of Mixtures

• Solutions: Homogeneous mixtures with very small particles that do not settle out or scatter light. Example: saline solution (NaCl in water).

• Colloids (emulsions, gels): Heterogeneous mixtures with intermediate-sized particles that do not settle easily and scatter light. Examples: milk, honey.

• Suspensions: Heterogeneous mixtures with large particles that tend to settle out over time and may scatter light. Examples: sand in water, blood.

Solutions: Solvent, Solute, and Concentration

Solutions consist of a solvent (the substance present in the greatest amount, usually water in biological systems) and one or more solutes (substances dissolved in the solvent).

• Concentration (%): Grams of solute per 100 mL of solution.

• Molarity (M): Number of moles of solute per liter of solution. One mole contains particles (Avogadro’s number).

Example: 0.9 g NaCl per 100 mL H2O is a 0.9% saline solution.

Chemical Bonds

Electron Shells and Energy Levels

Atoms consist of a nucleus surrounded by electrons arranged in shells or energy levels. Each shell can hold a specific maximum number of electrons, and shells are filled from the innermost to the outermost.

• 1st shell: up to 2 electrons

• 2nd shell: up to 8 electrons

• 3rd shell: up to 8 electrons

• 4th shell: more than 8 electrons

Electrons in the outermost shell (valence shell) determine an atom’s chemical reactivity.

The Octet Rule and Reactivity

The octet rule states that atoms are most stable when they have eight electrons in their valence shell (except for the first shell, which is stable with two electrons). Atoms with incomplete valence shells are reactive, while those with complete shells (noble gases) are inert.

• Inert elements: Full valence shell, nonreactive (e.g., helium, neon).

• Reactive elements: Incomplete valence shell, tend to gain, lose, or share electrons to achieve stability.

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, resulting in the formation of oppositely charged ions that attract each other.

• Cation: Atom that loses electrons, becoming positively charged (e.g., Na+).

• Anion: Atom that gains electrons, becoming negatively charged (e.g., Cl-).

Example: Sodium (Na) donates an electron to chlorine (Cl), forming NaCl (table salt).

Covalent Bonds

Covalent bonds involve the sharing of electrons between atoms to achieve stability. The number of shared electron pairs determines whether the bond is single, double, or triple.

• Single bond: One pair of electrons shared

• Double bond: Two pairs of electrons shared

• Triple bond: Three pairs of electrons shared

Electronegativity and Bond Polarity

The ability of an atom to attract shared electrons is called electronegativity. Atoms with high electronegativity (e.g., oxygen) attract electrons more strongly, while those with low electronegativity (e.g., hydrogen) tend to lose electrons. Carbon is considered electroneutral because it neither strongly attracts nor loses electrons.

Polar and Nonpolar Covalent Molecules

Nonpolar molecules have equal sharing of electrons, resulting in a balanced charge distribution (e.g., O2, CO2). Polar molecules have unequal sharing, creating partial positive (δ+) and negative (δ-) charges (e.g., H2O).

• Nonpolar: Linear, electrically balanced

• Polar: Bent or nonlinear, electrically unbalanced

Hydrogen Bonds

Hydrogen bonds are weak attractions between the positive (δ+) region of one polar molecule and the negative (δ-) region of another. They are important in maintaining the structure of water, proteins, and nucleic acids.

• Responsible for water’s surface tension

• Stabilize the 3D structure of large biological molecules

Bond Strength and Energy

The strength of a chemical bond is determined by the energy required to break it. In general:

• Hydrogen bonds: weakest

• Ionic bonds: intermediate

• Covalent bonds: strongest (triple > double > single)

Water’s polar nature allows for extensive hydrogen bonding, making it a unique and vital molecule in biological systems.

Chemical Reactions

Types of Chemical Reactions

Chemical reactions involve the formation, rearrangement, or breaking of chemical bonds. They are represented by chemical equations that show the reactants and products.

• Synthesis (combination) reactions: Two or more substances combine to form a larger, more complex molecule. Example: (anabolic)

• Decomposition reactions: A complex molecule is broken down into smaller components. Example: (catabolic)

• Exchange (displacement) reactions: Bonds are both broken and formed, resulting in new substances. Example:

• Oxidation-reduction (redox) reactions: Special exchange reactions involving the transfer of electrons between reactants.

Stoichiometry and Chemical Equations

Stoichiometry refers to the quantitative relationships between reactants and products in a chemical reaction. Chemical equations must be balanced to reflect the conservation of mass.

• Example:

Energy Flow in Chemical Reactions

Chemical reactions can either release or absorb energy:

• Exergonic reactions: Release energy; products have less potential energy than reactants; usually catabolic or oxidative.

• Endergonic reactions: Absorb energy; products have more potential energy than reactants; usually anabolic.

Summary Table: Types of Mixtures

Key Terms

• Mixture

• Solution

• Colloid

• Suspension

• Solvent

• Solute

• Molarity

• Avogadro’s number

• Chemical bond

• Ionic bond

• Covalent bond

• Hydrogen bond

• Electronegativity

• Polar molecule

• Nonpolar molecule

• Exergonic reaction

• Endergonic reaction