Chapter 3: Molecules and Compounds – Study Notes for General Chemistry

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Chapter 3: Molecules and Compounds

Introduction to Molecules and Compounds

When elements combine, they form compounds with properties distinct from their constituent elements. For example, hydrogen and oxygen gases react to form water, a compound with unique characteristics. Understanding the types of chemical bonds, chemical formulas, and nomenclature is essential for describing and predicting the behavior of compounds.

Hydrogen and oxygen reaction forming water, with periodic table and molecular models

Properties of Hydrogen, Oxygen, and Water

The physical and chemical properties of compounds differ significantly from those of the elements that compose them. For instance, hydrogen and oxygen are both gases at room temperature, while water is a liquid. Hydrogen is explosive, oxygen is necessary for combustion, and water is used to extinguish flames.

Selected Properties	Hydrogen	Oxygen	Water
Boiling Point	-253 °C	-183 °C	100 °C
State at Room Temperature	Gas	Gas	Liquid
Flammability	Explosive	Necessary for combustion	Used to extinguish flame

Table comparing properties of hydrogen, oxygen, and water

Mixtures vs. Compounds

Mixtures contain substances in variable ratios, while compounds have fixed ratios of elements. For example, a mixture of hydrogen and oxygen can have any proportion, but water always contains two hydrogen atoms for every oxygen atom.

Comparison of hydrogen and oxygen mixture versus water as a compound

Types of Chemical Bonds

Ionic and Covalent Bonds

Chemical bonds hold atoms together in compounds due to attractions between charged particles. There are two main types:

Ionic bonds: Occur between metals and nonmetals, involving the transfer of electrons. The resulting species is an ionic compound.
Covalent bonds: Occur between nonmetals, involving the sharing of electrons. The resulting species is a molecular compound.

Formation of an ionic compound (sodium chloride)

Chemical Formulas and Molecular Models

Types of Chemical Formulas

Chemical formulas indicate the elements present and their ratios in a compound. The main types are:

Empirical formula: Shows the simplest whole-number ratio of atoms.
Molecular formula: Shows the actual number of atoms of each element.
Structural formula: Shows how atoms are connected.

Molecular models provide visual representations:

Ball-and-stick model: Atoms as balls, bonds as sticks, showing shape.
Space-filling model: Shows the relative sizes and how atoms fill space.

Name of Compound	Empirical Formula	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Benzene	CH	C6H6	Hexagonal ring	Model	Model
Acetylene	CH	C2H2	Linear	Model	Model
Glucose	CH2O	C6H12O6	Ring/chain	Model	Model
Ammonia	NH3	NH3	Pyramidal	Model	Model

Table of empirical, molecular, and structural formulas with models

Atomic-Level View of Elements and Compounds

Classification of Pure Substances

Pure substances are classified as elements or compounds. Elements can be atomic (e.g., Ne) or molecular (e.g., O2). Compounds can be molecular (e.g., H2O) or ionic (e.g., NaCl).

Classification of elements and compounds at the atomic level

Molecular Elements

Some elements exist as molecules rather than single atoms. Most molecular elements are diatomic (e.g., H2, N2, O2, F2, Cl2, Br2, I2), while some are polyatomic (e.g., P4, S8).

Periodic table highlighting molecular elements

Molecular vs. Ionic Compounds

Molecular compounds are composed of nonmetals bonded covalently, while ionic compounds consist of metals and nonmetals bonded ionically. The basic unit of an ionic compound is the formula unit.

Comparison of molecular and ionic compounds

Writing and Naming Ionic Compounds

Formation of Ions and Ionic Compounds

Main group metals lose electrons to form cations, and nonmetals gain electrons to form anions. The charges are predictable based on their position in the periodic table.

Periodic table showing elements that form ions with predictable charges

Rules for Writing Ionic Formulas

To write the formula for an ionic compound:

Write the symbol for the metal cation and nonmetal anion.
Balance the charges by adjusting subscripts.
Ensure the smallest whole-number ratio of ions.

Naming Ionic Compounds

Ionic compounds are named by stating the cation (metal) first, followed by the base name of the anion (nonmetal) with the suffix -ide.

Naming convention for ionic compounds with one type of ion

Nonmetal	Symbol for Ion	Base Name	Anion Name
Fluorine	F-	fluor	Fluoride
Chlorine	Cl-	chlor	Chloride
Bromine	Br-	brom	Bromide
Iodine	I-	iod	Iodide
Oxygen	O2-	ox	Oxide
Sulfur	S2-	sulf	Sulfide
Nitrogen	N3-	nitr	Nitride
Phosphorus	P3-	phosph	Phosphide

Table of common monoatomic anions

Naming Ionic Compounds with Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms. Their names must be memorized and are used directly in compound names.

Name	Formula	Name	Formula
Acetate	C2H3O2-	Hypochlorite	ClO-
Carbonate	CO32-	Chlorite	ClO2-
Nitrate	NO3-	Sulfate	SO42-
Phosphate	PO43-	Ammonium	NH4+

Table of common polyatomic ions

Naming Ionic Compounds with Metals of Variable Charge

Some metals form more than one type of cation. Their charge is indicated in Roman numerals in parentheses after the metal name.

Naming convention for ionic compounds with metals of variable charge

Metal	Ion	Name	Older Name
Iron	Fe2+	Iron(II)	Ferrous
Iron	Fe3+	Iron(III)	Ferric
Copper	Cu+	Copper(I)	Cuprous
Copper	Cu2+	Copper(II)	Cupric

Table of metals that form cations with different charges

Naming Hydrated Ionic Compounds

Hydrates are ionic compounds containing a specific number of water molecules. Prefixes indicate the number of water molecules (e.g., hexahydrate for six).

Hydrated vs. anhydrous cobalt(II) chloride

Naming Molecular Compounds

Rules for Naming Molecular Compounds

Molecular compounds are named using prefixes to indicate the number of each atom. The first element is named fully; the second element uses the base name plus -ide. The prefix mono- is not used for the first element.

Naming convention for molecular compounds

Prefix	Meaning
mono-	1
di-	2
tri-	3
tetra-	4
penta-	5
hexa-	6
hepta-	7
octa-	8
nona-	9
deca-	10

Table of prefixes for naming molecular compounds

Naming Acids

Binary Acids and Oxyacids

Acids are compounds that release H+ in water. Binary acids consist of hydrogen and a nonmetal; their names begin with hydro- and end with -ic acid (e.g., HCl is hydrochloric acid). Oxyacids contain hydrogen and an oxyanion; if the oxyanion ends in -ite, the acid ends in -ous acid; if it ends in -ate, the acid ends in -ic acid.

Naming conventions for binary acids and oxyacids

Organic Compounds and Hydrocarbons

Classification of Organic Compounds

Organic compounds contain carbon and hydrogen, often with other elements. They are classified as hydrocarbons (alkanes, alkenes, alkynes) or functionalized hydrocarbons (alcohols, ethers, etc.).

Classification of organic and inorganic compounds

Families of Organic Compounds

Family	Name Ending	General Formula	Example	Name	Occurrence/Use
Alcohols	-ol	R–OH	CH3CH2OH	Ethanol	Alcohol in beverages
Ethers	ether	R–O–R'	CH3CH2OCH2CH3	Diethyl ether	Laboratory solvent
Aldehydes	-al	R–CHO	CH3CHO	Ethanal	Perfumes, flavors
Ketones	-one	R–CO–R'	CH3COCH3	Propanone	Nail polish remover
Carboxylic acids	-oic acid	R–COOH	CH3COOH	Acetic acid	Vinegar
Amines	-amine	RNH2	CH3CH2NH2	Ethyl amine	Smell of rotten fish

Table of families of organic compounds

Formula Mass and Molar Mass

Definitions and Calculations

The formula mass is the average mass of a molecule or formula unit in atomic mass units (amu). The molar mass is the mass of one mole of a substance in grams per mole (g/mol). These values are essential for converting between mass and number of particles.

Using Molar Mass to Count Molecules

Molar mass allows chemists to count molecules by weighing a sample. For example, the number of CO2 molecules in a given mass can be calculated using the molar mass and Avogadro's number.

Dry ice (solid CO2) used in molar mass calculations

Composition of Compounds

Mass Percent Composition

Mass percent composition expresses the proportion of each element in a compound by mass. It is calculated as:

Conversion Factors from Chemical Formulas

Chemical formulas provide relationships between the amounts of elements and compounds, allowing for stoichiometric calculations.

Determining Chemical Formulas from Experimental Data

Empirical and Molecular Formulas

The empirical formula gives the simplest ratio of atoms, while the molecular formula gives the actual number. To determine these from experimental data:

Find the mass of each element.
Convert masses to moles.
Write a pseudoformula based on mole ratios.
Divide by the smallest subscript to get whole numbers.
If necessary, multiply to obtain whole-number subscripts.

Fractional Subscript	Multiply by This
0.20	5
0.25	4
0.33	3
0.40	5
0.50	2
0.66	3
0.75	4
0.80	5

$Table for converting fractional subscripts to whole numbers$

Combustion Analysis

Combustion analysis is used to determine the empirical formula of organic compounds by burning a sample and measuring the masses of CO2 and H2O produced.

Apparatus for combustion analysis

Summary

Compounds have fixed ratios of elements and unique properties.
Chemical bonds can be ionic or covalent.
Chemical formulas and models represent compounds in various ways.
Naming conventions differ for ionic, molecular, and acid compounds.
Empirical and molecular formulas can be determined from experimental data.
Organic compounds are classified by functional groups and structure.