Molecules and Compounds: Structure, Classification, and Nomenclature

Chapter 5: Molecules and Compounds

Introduction to Compounds

Chemical compounds are substances formed from two or more elements combined in fixed proportions. Their properties are often very different from those of the constituent elements.

• Compound: A substance composed of two or more elements chemically bonded in definite ratios.

• Example: Sucrose (table sugar) contains carbon, hydrogen, and oxygen atoms, but its properties differ greatly from those of elemental carbon, hydrogen, or oxygen.

• Example: Sodium chloride (table salt) is formed from sodium (a reactive metal) and chlorine (a poisonous gas), yet the compound is safe to eat.

Elements vs. Compounds

Most substances encountered in daily life are compounds, not elements. Elements are rare in nature as isolated atoms.

• Compound: Elements combine in fixed, definite proportions.

• Mixture: Elements can be present in any proportion.

Law of Constant Composition

Joseph Proust established that all samples of a given compound have the same proportions of their constituent elements.

• Law of Constant Composition: All samples of a compound have the same elemental ratios by mass.

• Example (Water): Decomposing 18.0 g of water yields 16.0 g oxygen and 2.0 g hydrogen. Mass ratio: $\frac{16.0\,g\,O}{2.0\,g\,H} = 8.0:1$

• Example (Ammonia): Decomposing 17.0 g of ammonia yields 14.0 g nitrogen and 3.0 g hydrogen. Mass ratio: $\frac{14.0\,g\,N}{3.0\,g\,H} = 4.7:1$

Chemical Formulas

Chemical formulas represent the elements present in a compound and the relative number of atoms of each.

• Format: Element symbols with subscripts indicating the number of atoms (subscript of 1 is omitted).

• Examples:

  • Water: $H_2O$ (2 hydrogen, 1 oxygen)

  • Table salt: $NaCl$ (1 sodium, 1 chlorine)

  • Carbon dioxide: $CO_2$ (1 carbon, 2 oxygen)

  • Sucrose: $C_{12}H_{22}O_{11}$ (12 carbon, 22 hydrogen, 11 oxygen)

• Changing subscripts creates a different compound (e.g., $CO$ vs. $CO_2$).

Order of Elements in Chemical Formulas

The most metallic element is listed first in a chemical formula. For compounds without metals, the more metal-like nonmetal is listed first.

• Metals are on the left side of the periodic table; nonmetals on the right.

• Within a group, elements lower in the column are more metal-like.

• Examples: $NaCl$ (not $ClNa$), $NO_2$ (not $ON_2$), $SO_2$ (not $O_2S$).

Polyatomic Ions in Chemical Formulas

Some chemical formulas contain groups of atoms that act as a unit, called polyatomic ions. Parentheses and subscripts indicate the number of such groups.

• Example: $Mg(NO_3)_2$ contains 1 magnesium atom, 2 nitrate groups ($NO_3^-$).

• To find the total number of each atom, multiply the subscript outside the parentheses by the subscript inside.

• Calculation for $Mg(NO_3)_2$:

  • Mg: $1 \times 1 = 1$

  • N: $1 \times 2 = 2$

  • O: $3 \times 2 = 6$

Types of Chemical Formulas

There are several ways to represent compounds:

• Empirical formula: Shows the simplest whole-number ratio of atoms (e.g., $HO$ for hydrogen peroxide).

• Molecular formula: Shows the actual number of atoms (e.g., $H_2O_2$ for hydrogen peroxide).

• Structural formula: Shows how atoms are connected using lines for bonds.

• Molecular models: Ball-and-stick and space-filling models represent the 3D arrangement of atoms.

Classification of Elements and Compounds

Substances can be classified as elements or compounds, and further as atomic, molecular, ionic, or molecular compounds.

• Elements:

  • Atomic elements: Exist as single atoms (e.g., mercury).

  • Molecular elements: Exist as molecules, often diatomic (e.g., $Cl_2$, $O_2$).

• Compounds:

  • Molecular compounds: Formed from nonmetals; basic units are molecules (e.g., $CO_2$).

  • Ionic compounds: Contain cations (usually metals) and anions (usually nonmetals); basic units are formula units (e.g., $NaCl$).

Formulas for Ionic Compounds

Ionic compounds always contain positive (cation) and negative (anion) ions. The sum of charges must be zero.

• Write the symbol and charge for each ion.

• Use the magnitude of each ion's charge as the subscript for the other ion.

• Reduce subscripts to the smallest whole-number ratio.

• Example: $Al^{3+}$ and $O^{2-}$ combine to form $Al_2O_3$.

Common Polyatomic Ions

Polyatomic ions are groups of atoms with an overall charge. Familiarity with common polyatomic ions is essential.

Ion Name	Formula
Hydroxide	OH-
Nitrate	NO3-
Sulfate	SO42-
Carbonate	CO32-
Phosphate	PO43-
Ammonium	NH4+
Bicarbonate	HCO3-
Chlorate	ClO3-
Additional info: Table inferred from context and common ions.

Nomenclature: Naming Compounds

Chemists use systematic rules to name compounds. The name can be determined from the formula and vice versa.

• Common names: Some compounds have widely used names (e.g., water for $H_2O$).

• Systematic names: Follow specific rules based on the type of compound.

Naming Ionic Compounds

Ionic compounds are named based on the metal and nonmetal present. The type of metal determines the naming convention.

• Type I (invariant charge): Metal forms only one type of ion.

  • Name: [metal] [base name of nonmetal + -ide]

  • Example: $NaCl$ is sodium chloride.

• Type II (variable charge): Metal forms more than one type of ion.

  • Name: [metal] (Roman numeral for charge) [base name of nonmetal + -ide]

  • Example: $FeCl_3$ is iron(III) chloride.

• Polyatomic ions: Use the name of the polyatomic ion in the compound name.

  • Example: $KNO_3$ is potassium nitrate.

  • Example: $Fe(OH)_2$ is iron(II) hydroxide.

  • Example: $NH_4NO_3$ is ammonium nitrate.

Naming Oxyanions and Oxyacids

Oxyanions are polyatomic ions containing oxygen. Their names depend on the number of oxygen atoms.

• Two ions: -ate (more oxygen), -ite (less oxygen)

  • $NO_3^-$: nitrate

  • $NO_2^-$: nitrite

  • $SO_4^{2-}$: sulfate

  • $SO_3^{2-}$: sulfite

• More than two ions: Use prefixes hypo- (least oxygen) and per- (most oxygen)

  • $ClO^-$: hypochlorite

  • $ClO_2^-$: chlorite

  • $ClO_3^-$: chlorate

  • $ClO_4^-$: perchlorate

Naming Molecular Compounds

Molecular compounds are formed from two or more nonmetals. Prefixes indicate the number of each atom.

• Prefixes: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10)

• Format: [prefix][name of first element] [prefix][base name of second element + -ide]

• Example: $CO_2$ is carbon dioxide (mono- omitted for first element).

• Example: $N_2O$ is dinitrogen monoxide (mono- drops the 'o' before oxide).

Naming Acids

Acids are molecular compounds that release hydrogen ions in water. They are categorized as binary acids or oxyacids.

• Binary acids: Contain hydrogen and one nonmetal.

  • Name: hydro-[base name of nonmetal + -ic] acid

  • Example: $HCl$ (aq) is hydrochloric acid.

  • Example: $H_2S$ (aq) is hydrosulfuric acid.

• Oxyacids: Contain hydrogen, a nonmetal, and oxygen (as part of a polyatomic ion).

  • If oxyanion ends in -ate: [base name of oxyanion + -ic] acid

  • If oxyanion ends in -ite: [base name of oxyanion + -ous] acid

  • Example: $HNO_3$ (aq) is nitric acid ($NO_3^-$ is nitrate).

  • Example: $HNO_2$ (aq) is nitrous acid ($NO_2^-$ is nitrite).

Formula Mass

The formula mass of a compound is the sum of the atomic masses of all atoms in its chemical formula.

• Formula: $\text{Formula mass} = \sum (\text{number of atoms of each element} \times \text{atomic mass of element})$

• Example: For $H_2O$, $2 \times 1.008 + 1 \times 16.00 = 18.016$ amu

Chemistry in the Environment: Acid Rain

Acid rain forms when rainwater mixes with air pollutants such as $NO_x$ and $SO_x$ that form acids.

• $NO_2$ and $NO_3$ from vehicle emissions combine with water to form $HNO_3$ (nitric acid).

• $SO_2$ from coal combustion forms $H_2SO_4$ (sulfuric acid).

• Acid rain harms aquatic life, damages trees, and erodes buildings.

Chemical Skills Learning Objectives

• Restate and apply the law of constant composition.

• Write chemical formulas and determine the number of each type of atom.

• Classify elements and compounds as atomic, molecular, or ionic.

• Write formulas for ionic compounds.

• Name binary ionic compounds (Type I and II), compounds with polyatomic ions, molecular compounds, binary acids, and oxyacids.

• Calculate formula mass.

Summary Table: Types of Compounds and Naming Rules

Type	Formula Example	Naming Rule
Type I Ionic	NaCl	Metal + nonmetal(-ide)
Type II Ionic	FeCl3	Metal (Roman numeral) + nonmetal(-ide)
Polyatomic Ionic	KNO3	Metal + polyatomic ion
Molecular	CO2	Prefix + element + prefix + base(-ide)
Binary Acid	HCl (aq)	hydro + base(-ic) acid
Oxyacid	HNO3 (aq)	base(-ic/-ous) acid

Additional info: Some tables and lists have been inferred and expanded for completeness and clarity.