The Chemical Context of Life: Elements, Atoms, and Chemical Bonds

• Chapter 2: The Chemical Context of Life

Introduction

Understanding the chemical basis of life is essential for studying biology. Living organisms are composed of matter, which is made up of elements and compounds. The structure and interactions of atoms determine the properties of matter and the molecules essential for life.

Concept 2.1: Matter, Elements, and Compounds

Definition of Matter

• Matter is anything that takes up space and has mass.

• Mass is the amount of matter in an object; weight is the force of gravity acting on that mass.

Elements and Compounds

• Element: A substance that cannot be broken down to other substances by chemical reactions.

• Compound: A substance consisting of two or more elements in a fixed ratio. Compounds have emergent properties distinct from their constituent elements.

The Elements of Life

• About 20–25% of the 92 natural elements are essential for life.

• Major elements: Carbon, hydrogen, oxygen, and nitrogen make up about 96% of living matter.

• Other important elements: Calcium, phosphorus, potassium, sulfur, and trace elements are also required in smaller amounts.

Concept 2.2: Atomic Structure and Properties

Atoms and Subatomic Particles

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

• Composed of protons (positive charge), neutrons (no charge), and electrons (negative charge).

• Protons and neutrons are found in the nucleus; electrons form a cloud around the nucleus.

Atomic Number, Mass Number, and Atomic Mass

• Atomic number: Number of protons in the nucleus (unique to each element).

• Mass number: Sum of protons and neutrons in the nucleus.

• Atomic mass: Total mass of an atom, nearly equal to the mass number (measured in daltons).

Energy Levels of Electrons

• Energy: The capacity to cause change.

• Potential energy: Energy due to location or structure; electrons have different potential energies depending on their distance from the nucleus.

• Electrons occupy specific energy levels or shells.

Electron Distribution and Chemical Properties

• The arrangement of electrons in shells determines an atom's chemical behavior.

• The periodic table organizes elements by electron configuration.

Valence Electrons and Orbitals

• Valence electrons: Electrons in the outermost shell; determine chemical reactivity.

• Atoms with full valence shells are chemically inert.

• Orbitals: Three-dimensional spaces where electrons are found 90% of the time; each can hold up to two electrons.

Concept 2.3: Chemical Bonds and Molecular Interactions

Chemical Bonds

• Atoms with incomplete valence shells can share or transfer electrons, forming chemical bonds.

• Main types of bonds: Covalent bonds, ionic bonds, hydrogen bonds, and van der Waals interactions.

Covalent Bonds

• Covalent bond: Sharing of a pair of valence electrons between two atoms.

• Single bond: Sharing of one pair of electrons; double bond: sharing of two pairs.

• Structural formula: Shows arrangement of atoms and bonds (e.g., H—H, O═O).

• Molecular formula: Indicates the number of each type of atom (e.g., H2).

Electronegativity and Polarity

• Electronegativity: An atom's attraction for electrons in a covalent bond.

• Nonpolar covalent bond: Electrons are shared equally.

• Polar covalent bond: Electrons are shared unequally, resulting in partial charges.

Ionic Bonds

• Formed when one atom strips electrons from another, creating ions.

• Cation: Positively charged ion; anion: negatively charged ion.

• Attraction between cations and anions forms an ionic bond.

• Ionic compounds (salts): Compounds formed by ionic bonds, often crystalline in structure.

Weak Chemical Interactions

• Weak bonds (hydrogen bonds, van der Waals interactions) are crucial for the structure and function of large biological molecules.

• Weak bonds are reversible, allowing dynamic molecular interactions.

Hydrogen Bonds

• Form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom (often oxygen or nitrogen).

Van der Waals Interactions

• Occur when transiently positive and negative regions of molecules attract each other.

• Individually weak, but collectively significant in biological systems (e.g., gecko toe adhesion).

Concept 2.4: Chemical Reactions

Making and Breaking Bonds

• Chemical reactions: Processes that make and break chemical bonds.

• Reactants: Starting molecules; products: resulting molecules.

Photosynthesis as a Chemical Reaction

• Photosynthesis converts carbon dioxide and water into glucose and oxygen using sunlight.

$6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2$

Chemical Equilibrium

• Equilibrium is reached when forward and reverse reactions occur at the same rate.

• At equilibrium, the concentrations of reactants and products remain constant.

Additional info: This guide covers the foundational chemical principles necessary for understanding biological molecules and processes, including atomic structure, types of chemical bonds, and the nature of chemical reactions in living systems.