Chemical Level of Organization: Atoms, Molecules, and Compounds in Anatomy & Physiology

Section 1: Atoms, Molecules, and Compounds

Introduction

The chemical level of organization is fundamental to understanding the structure and function of the human body. This section explores the properties of atoms, molecules, and compounds, and their roles in physiological processes.

Learning Outcomes

• Define an atom and describe its properties.

• Describe atomic structure and the functions of subatomic particles.

• Explain chemical bonds and the formation of molecules and compounds.

• Visualize molecular interactions such as hydrogen bonding in water.

Module 2.1: Atoms as the Basic Particles of Matter

Atoms and Matter

Atoms are the smallest units of matter that retain the properties of an element. Matter is anything that occupies space and has mass.

• Atom: Smallest unit of a chemical element.

• Matter: Anything that has mass and takes up space.

• Mass: Quantity of matter in an object; on Earth, mass is equivalent to weight.

Module 2.1: Atomic Structure

Subatomic Particles

Atoms are composed of subatomic particles: protons, neutrons, and electrons.

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles found in the nucleus.

• Electrons: Negatively charged particles orbiting the nucleus.

Atomic number is the number of protons in an atom, which defines the element.

Mass number is the sum of protons and neutrons in the nucleus.

Module 2.2: Atoms, Elements, and Isotopes

Elements

An element is a substance composed of atoms with the same atomic number. Elements are represented by chemical symbols (e.g., H for hydrogen, O for oxygen).

• Principal elements in the human body: Oxygen, Carbon, Hydrogen, Nitrogen, Calcium, Phosphorus, Potassium, Sodium, Chlorine, Magnesium, Sulfur, Iron, Iodine.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Atomic Mass and Atomic Weight

• Atomic mass: Mass of a specific isotope.

• Atomic weight: Average mass of all isotopes of an element.

Formula:

$\text{Atomic mass} = \text{Number of protons} + \text{Number of neutrons}$

Module 2.3: Electrons and Energy Levels

Electron Shells

Electrons occupy specific energy levels or shells around the nucleus. The arrangement of electrons determines the chemical properties of an element.

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

• Octet rule: Atoms tend to gain, lose, or share electrons to achieve a full outer shell (usually 8 electrons).

Ions

Atoms become ions when they gain or lose electrons.

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

Module 2.4: Chemical Bonds

Ionic Bonds

Ionic bonds are formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions that attract each other.

• Example: Sodium chloride (NaCl) forms when sodium donates an electron to chlorine.

Covalent Bonds

Covalent bonds are formed when atoms share electrons.

• Single covalent bond: One pair of electrons shared.

• Double covalent bond: Two pairs of electrons shared.

• Nonpolar covalent bond: Electrons shared equally.

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

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom in one molecule and an electronegative atom in another molecule. They are important in water and biological molecules.

Module 2.5: States of Matter

Solid, Liquid, and Gas

Matter exists as solids, liquids, or gases, depending on temperature and pressure.

• Solid: Definite shape and volume.

• Liquid: Definite volume, no definite shape.

• Gas: No definite shape or volume.

Principal Elements of the Human Body

Table: Major Elements and Their Functions

Element	Symbol	Body %	Function
Oxygen	O	65.0	Component of water; essential for cellular respiration
Carbon	C	18.6	Backbone of organic molecules
Hydrogen	H	9.7	Component of water and most organic molecules
Nitrogen	N	3.2	Component of proteins and nucleic acids
Calcium	Ca	1.8	Structural component of bones and teeth; involved in signaling
Phosphorus	P	1.0	Component of nucleic acids and ATP
Potassium	K	0.4	Important for nerve function and muscle contraction
Sodium	Na	0.2	Important for fluid balance and nerve function
Chlorine	Cl	0.2	Helps maintain fluid balance
Magnesium	Mg	0.06	Cofactor for many enzymes
Sulfur	S	0.04	Component of proteins
Iron	Fe	0.007	Component of hemoglobin
Iodine	I	0.0002	Component of thyroid hormones

Summary Table: Types of Chemical Bonds

Bond Type	Description	Example
Ionic	Transfer of electrons between atoms	NaCl (sodium chloride)
Covalent	Sharing of electrons between atoms	H2O (water), O2 (oxygen gas)
Hydrogen	Weak attraction between molecules	Between water molecules

Additional info:

• Understanding atomic structure and chemical bonding is essential for grasping physiological processes such as metabolism, cellular respiration, and molecular transport.

• Isotopes can be used in medical imaging and diagnostic techniques (e.g., radioactive tracers).