Chapter 3: Molecules and Compounds – General Chemistry Study Notes

Chapter 3: Molecules and Compounds

Introduction to Compounds

Chemical compounds are substances formed from two or more elements that are chemically bonded together. These compounds possess unique chemical and physical properties that are distinct from the properties of the individual elements from which they are composed.

• Example: Water (H2O) is a liquid at room temperature, while its constituent elements, hydrogen (H2) and oxygen (O2), are both gases.

• Key Point: The properties of a compound cannot be predicted simply by knowing the properties of its elements.

Property	Hydrogen (H2)	Oxygen (O2)	Water (H2O)
Boiling Point	−253°C	−183°C	100°C
State at Room Temp	Gas	Gas	Liquid
Flammability	Explosive	Necessary for combustion	Used to extinguish flames

Kinds of Compounds

Molecular Compounds

Molecular compounds are composed of atoms held together by covalent bonds, where pairs of electrons are shared between atoms.

• Nonmetals are bonded to each other.

• Exist as discrete units called molecules.

• Example: Propane (C3H8), Water (H2O)

Ionic Compounds

Ionic compounds are composed of positively and negatively charged ions held together by electrostatic attraction (ionic bonds).

• Usually formed between a metal (cation) and a nonmetal (anion).

• Exist as a repeating structure called a crystal lattice.

• Example: Sodium chloride (NaCl)

Key Terms and Definitions

Molecular Formula

• Shows the exact number and type of atoms present in one molecule of a compound.

• Applies to molecular compounds only.

• Example: C6H12O6 (glucose)

Empirical Formula

• Gives the simplest whole-number ratio of elements in a compound.

• Example: CH2O is the empirical formula for glucose.

Formula Unit

• The smallest electrically neutral unit within the crystal of an ionic compound.

• Example: NaCl (one Na+ and one Cl−)

Representing Chemical Formulas

Chemical formulas use elemental symbols to represent atoms, with subscripts to indicate the number of each atom present (subscript '1' is omitted).

• Molecular Formula: CH4 (methane)

• Structural Formula: Shows how atoms are connected (e.g., H–C–H)

• Ball-and-Stick Model: Shows 3D geometry of the molecule.

• Space-Filling Model: Most accurate representation of molecular size and shape.

Atomic-Level Classification

Pure substances can be classified as elements or compounds, and further as atomic, molecular, or ionic based on their structure.

• Elements: Can be atomic (e.g., Ne) or molecular (e.g., O2).

• Compounds: Can be molecular (e.g., H2O) or ionic (e.g., NaCl).

Type	Example	Structure
Atomic Element	Ne	Single atoms
Molecular Element	O2	Diatomic molecules
Molecular Compound	H2O	Discrete molecules
Ionic Compound	NaCl	Crystal lattice of ions

Summary Table: Types of Chemical Formulas

Formula Type	Description	Example
Molecular Formula	Exact number of each atom in a molecule	H2O
Empirical Formula	Simplest whole-number ratio of atoms	CH2O
Formula Unit	Smallest neutral unit in an ionic compound	NaCl

Key Points for Study

• Compounds have properties distinct from their constituent elements.

• Molecular compounds are made of nonmetals and exist as molecules.

• Ionic compounds are made of metals and nonmetals, forming crystal lattices.

• Chemical formulas use symbols and subscripts to represent composition.

• Empirical formulas show the simplest ratio; molecular formulas show the actual number of atoms.

• Formula units are used for ionic compounds.

Additional info: The notes above are based on the first several slides of a General Chemistry lecture on molecules and compounds, focusing on definitions, classification, and representation of chemical substances. Later sections would likely cover naming, formulas, and calculations in more detail.