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

Chapter 2: The Chemical Context for Life

Introduction

Understanding the chemical basis of life is essential for studying biology. Atoms and molecules form the foundation of all biological structures and processes. This chapter explores the nature of elements, atomic structure, chemical bonds, and the interactions that drive biological chemistry.

Elements and Atoms

Elements

• Element: A substance that cannot be broken down into other substances by chemical means.

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

Element	Life (humans)	Atmosphere	Earth's crust
Oxygen (O)	65%	21%	46%
Carbon (C)	18%	Trace	Trace
Hydrogen (H)	10%	Trace	Trace
Nitrogen (N)	3%	78%	Trace

Additional info: The atmosphere is mostly nitrogen, but humans require oxygen for survival.

Atoms: Structure and Terms

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

• Composed of subatomic particles:

  • Protons: Positive charge, found in the nucleus.

  • Neutrons: No charge, found in the nucleus.

  • Electrons: Negative charge, orbit the nucleus.

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

• Mass number: Sum of protons and neutrons.

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

The Periodic Table and Electron Shells

Periodic Table of Elements

• Organizes elements by atomic number and properties.

• Rows (periods) correspond to the number of electron shells.

• Columns (groups) indicate the number of valence electrons.

Electron Shells and Energy Levels

• Electrons occupy energy levels (shells) around the nucleus.

• First shell: up to 2 electrons (lowest energy).

• Second shell: up to 8 electrons (higher energy).

• Third shell: up to 8 electrons (even higher energy).

• Electrons fill the lowest available energy levels first.

Valence Electrons and Chemical Properties

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

• Atoms are most stable when their valence shell is full (octet rule).

• Atoms with incomplete valence shells tend to form bonds to achieve stability.

Chemical Bonds

Covalent Bonds

• Formed when two atoms share one or more pairs of electrons.

• Can be single, double, or triple bonds depending on the number of shared electron pairs.

• Structural formula: Shows how atoms are bonded (e.g., H—O—H for water).

Polar vs. Nonpolar Covalent Bonds

• Nonpolar covalent bond: Electrons are shared equally (e.g., H2, O2).

• Polar covalent bond: Electrons are shared unequally, creating partial charges (e.g., H2O).

Water	Methane	Carbon dioxide
Polar	Nonpolar	Nonpolar

Ionic Bonds

• Formed when one atom transfers an electron to another, creating ions.

• Cation: Positively charged ion (loses electron).

• Anion: Negatively charged ion (gains electron).

• Ionic compounds are held together by electrostatic attraction (e.g., NaCl).

Hydrogen Bonds

• Weak bonds between a hydrogen atom (covalently bonded to O, N, or F) and another electronegative atom.

• Important in stabilizing the structures of proteins and DNA.

Van der Waals Interactions

• Weak attractions between molecules due to transient local partial charges.

• Significant in large molecules and biological macromolecules.

Chemical Reactions

Making and Breaking Bonds

• Chemical reactions rearrange atoms into new molecules.

• Photosynthesis: An important chemical reaction in biology.

  • Equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

• Reversible reactions: Indicated by double arrows; can proceed in both directions.

Summary Table: Types of Chemical Bonds

Bond Type	Strength	Example
Covalent	Strong	H2O, CH4
Ionic	Moderate	NaCl
Hydrogen	Weak	Between water molecules
Van der Waals	Very weak	Between nonpolar molecules

Key Terms

• Element

• Atom

• Isotope

• Covalent bond

• Ionic bond

• Hydrogen bond

• Van der Waals interaction

• Valence electron

• Electronegativity

Additional info: Understanding these chemical principles is foundational for all subsequent topics in biology, including metabolism, genetics, and cell structure.