Chapter 2: The Chemical Context of Life – Atoms, Elements, and Chemical Bonds

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Introduction to Biology and Chemistry of Life

Overview of Life's Chemical Basis

All living organisms are composed of matter, which is made up of elements. Understanding the chemical context of life is fundamental to biology, as it explains how atoms and molecules interact to form the structures and processes essential for life.

Order, Energy, Growth, Reproduction, Evolution, Response to Environment, Regulation: These are key characteristics of living systems, all of which depend on chemical processes.
Basic Chemistry: Provides the foundation for understanding biological molecules and reactions.
Structure and Function: The arrangement of atoms determines the properties and functions of molecules in cells.

Learning Outcomes

Recognize that matter is composed of elements that cannot be broken down chemically, and atoms are the smallest unit of matter that retains the properties of an element.
Interpret an atom’s structure based on its atomic mass and atomic number.
Use an understanding of valence electrons to compare covalent, ionic, and hydrogen bonds, as well as van der Waals forces.
Explain an atom’s electronegativity based on its position in the periodic table and relate this to its binding properties.

Atoms

Structure and Properties of Atoms

An atom is the smallest unit of an element that retains its chemical properties. Atoms are composed of subatomic particles: protons, neutrons, and electrons.

Proton: Positively charged particle located in the nucleus.
Neutron: Neutrally charged particle located in the nucleus.
Electron: Negatively charged particle orbiting the nucleus in electron shells.

Particle	Charge	Mass?	Location?
Proton	+1	Yes	In nucleus
Neutron	0	Yes	In nucleus
Electron	-1	No	Orbit nucleus

Example: A helium atom has 2 protons, 2 neutrons, and 2 electrons.

Periodic Table

Organization and Biological Relevance

The periodic table organizes elements by increasing atomic number and similar chemical properties. Elements essential to life include oxygen, carbon, hydrogen, and nitrogen, which make up the majority of living matter.

Trace elements are required in very small amounts but are vital for life (e.g., iron, iodine).
The periodic table helps predict element behavior based on position (groups and periods).

Element	Approximate % of Human Body
Oxygen (O)	65%
Carbon (C)	18.5%
Hydrogen (H)	9.5%
Nitrogen (N)	3.3%
Trace elements	<0.01%

Atomic Number and Atomic Mass

Defining Atoms and Isotopes

Each element is defined by its atomic number (number of protons). The atomic mass (also called mass number) is the sum of protons and neutrons in the nucleus.

Atomic number (Z): Number of protons in the nucleus.
Atomic mass (A): Number of protons plus neutrons.
Atoms are electrically neutral when the number of protons equals the number of electrons.

Formula:

Example: Helium (He) has atomic number 2 and atomic mass 4.003.

Electron Configuration and Valence Electrons

Electron Shells and Chemical Reactivity

Electrons are arranged in shells around the nucleus. The valence electrons are those in the outermost shell and determine an atom’s chemical behavior.

The first shell holds up to 2 electrons; the second shell holds up to 8 electrons.
Atoms are most stable when their outer shell is full (octet rule).
Electron dot diagrams (Lewis structures) represent valence electrons as dots around the element symbol.

Example: Oxygen has 6 valence electrons; it needs 2 more to fill its outer shell.

Isotopes and Radioactivity

Variants of Elements and Their Uses

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different atomic masses. Some isotopes are radioactive and decay spontaneously, emitting energy.

Stable isotopes: Do not change over time.
Radioactive isotopes: Unstable; used in dating fossils and medical imaging.

Example: Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons).

Chemical Bonds

Types of Chemical Bonds

Atoms form chemical bonds to achieve stable electron configurations. The main types of bonds in biology are covalent, ionic, hydrogen bonds, and van der Waals interactions.

Covalent bonds: Atoms share pairs of valence electrons. Can be single, double, or triple bonds.
Ionic bonds: Electrons are transferred from one atom to another, creating charged ions (cations and anions) that attract each other.
Hydrogen bonds: Weak attractions between a hydrogen atom (partially positive) and an electronegative atom (often oxygen or nitrogen).
van der Waals interactions: Weak, temporary attractions due to shifting electron clouds.

Covalent Bonds: Polar vs. Nonpolar

Nonpolar covalent bond: Electrons are shared equally (e.g., O2, H2).
Polar covalent bond: Electrons are shared unequally due to differences in electronegativity (e.g., H2O).

Electronegativity is an atom’s ability to attract electrons in a bond. It increases across a period and decreases down a group in the periodic table.

Example: In water (H2O), oxygen is more electronegative than hydrogen, resulting in partial charges and hydrogen bonding.

Ionic Bonds

Formed when one atom donates an electron to another, resulting in oppositely charged ions.
Common in salts such as sodium chloride (NaCl).

Example: Na (sodium) loses an electron to become Na+; Cl (chlorine) gains an electron to become Cl-.

Hydrogen Bonds and van der Waals Interactions

Hydrogen bonds: Important in stabilizing the structure of water, proteins, and DNA.
van der Waals interactions: Weak, transient attractions that can stabilize molecules in close proximity.

Example: Hydrogen bonds hold water molecules together, contributing to water’s unique properties.

Summary Table: Types of Chemical Bonds

Bond Type	Mechanism	Relative Strength	Biological Example
Covalent	Sharing of electrons	Strongest	Peptide bonds in proteins
Ionic	Transfer of electrons	Strong (in dry conditions)	NaCl (table salt)
Hydrogen	Attraction between partial charges	Weak	Between water molecules
van der Waals	Temporary dipole attractions	Weakest	Lipid interactions in membranes

Key Terms

Element: Substance that cannot be broken down by chemical means.
Atom: Smallest unit of an element retaining its properties.
Isotope: Atoms of the same element with different numbers of neutrons.
Ion: Atom or molecule with a net electric charge due to loss or gain of electrons.
Electronegativity: Tendency of an atom to attract electrons in a bond.
Valence electrons: Electrons in the outermost shell, involved in bonding.