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

Chapter 2: The Chemical Context of Life

Introduction

This chapter explores the fundamental chemical principles that underlie biological processes. Understanding the nature of elements, atoms, and chemical bonds is essential for grasping how living organisms are structured and how they function at the molecular level.

Elements and Compounds

Definition and Properties

• Element: A substance that cannot be broken down into other substances by chemical reactions. Each element is defined by its number of protons.

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

• Emergent Properties: New characteristics that arise when elements combine to form compounds, such as the difference between sodium, chlorine, and sodium chloride.

Elements Essential for Life

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

• Four elements make up 96% of living matter: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).

• Other important elements include calcium (Ca), phosphorus (P), potassium (K), and sulfur (S).

• Trace elements are required in minute quantities (e.g., iron, zinc).

Element	Symbol	Percentage of Body Mass (including water)
Oxygen	O	65.0%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Calcium	Ca	1.5%
Phosphorus	P	1.0%
Potassium	K	0.4%
Sulfur	S	0.3%
Sodium	Na	0.2%
Chlorine	Cl	0.2%
Magnesium	Mg	0.1%

Atoms and Subatomic Particles

Structure of Atoms

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

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

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

• Atomic mass is measured in daltons (Da).

Atomic Number, Mass Number, and Isotopes

• Atomic number: Number of protons in the nucleus; defines the element.

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

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Radioactive isotopes have unstable nuclei and can be used in biological research and medicine (e.g., PET scans).

Energy Levels and Electron Distribution

Energy and Potential Energy

• Energy: The capacity to cause change.

• Potential energy: Energy possessed due to location or structure; electrons further from the nucleus have higher potential energy.

• Electrons occupy specific energy levels or shells; energy changes occur in discrete steps.

Electron Shells and Chemical Properties

• Electron distribution determines chemical behavior.

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

• Elements with full valence shells are inert (nonreactive).

Chemical Bonds

Covalent Bonds

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

• Can be single (one pair shared) or double (two pairs shared).

• Molecule: Two or more atoms held together by covalent bonds.

• Electronegativity: An atom's attraction for shared electrons; determines bond polarity.

• Nonpolar covalent bond: Electrons shared equally.

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

Ionic Bonds

• Ionic bond: Attraction between oppositely charged ions (cations and anions).

• Formed when electrons are transferred from one atom to another.

• Ionic compounds (salts): Stable in dry form, dissociate in water (e.g., NaCl).

Weak Chemical Interactions

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

• Allow for reversible interactions, important in processes like DNA replication and protein folding.

Hydrogen Bonds

• Form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom (usually O or N).

• Responsible for many properties of water and the structure of biomolecules.

Chemical Reactions and Equilibrium

Chemical Reactions

• Involve the making and breaking of chemical bonds.

• Reactants: Starting materials.

• Products: Resulting materials.

• All reactions are reversible to some extent.

Photosynthesis as a Chemical Reaction

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

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} \xrightarrow{\text{sunlight}} \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

Chemical Equilibrium

• Occurs when forward and reverse reactions proceed at the same rate.

• Relative concentrations of reactants and products remain constant at equilibrium.

Summary Table: Key Terms and Concepts

Term	Definition
Element	Substance that cannot be broken down by chemical means
Compound	Substance of two or more elements in a fixed ratio
Atom	Smallest unit of an element
Isotope	Atoms of the same element with different numbers of neutrons
Covalent bond	Sharing of electron pairs between atoms
Ionic bond	Attraction between oppositely charged ions
Hydrogen bond	Attraction between a hydrogen atom and an electronegative atom
Valence electrons	Electrons in the outermost shell
Chemical equilibrium	State where forward and reverse reactions occur at the same rate