Molecules and Compounds: Classification, Representation, and Nomenclature

Chapter 5: Molecules and Compounds Outline

Overview

This chapter introduces the classification of chemical compounds, their representation, and systematic methods for naming them. Understanding these foundational concepts is essential for further study in chemistry.

Classifying Compounds by Type

Types of Chemical Substances

• Atomic Element: A substance consisting of single atoms of one element (e.g., Aluminum, Al).

• Molecular Element: Elements that exist as molecules composed of two or more atoms of the same element (e.g., Chlorine, Cl2).

• Molecular Compound: Compounds formed from two or more nonmetals, held together by covalent bonds (e.g., Water, H2O).

• Ionic Compound: Compounds formed from metals and nonmetals, held together by ionic bonds (e.g., Sodium chloride, NaCl).

Key Point: The type of elements involved determines the classification and properties of the compound.

Classifying and Representing Compounds

Introduction to Chemical Compounds

Chemical compounds are substances composed of two or more different elements chemically bonded together. They can be classified based on the nature of their bonding and constituent elements.

• Ionic Compounds: Formed from metals and nonmetals; consist of cations and anions.

• Molecular Compounds: Formed from nonmetals; consist of molecules with covalent bonds.

Example: Table salt (NaCl) is an ionic compound, while sugar (C12H22O11) is a molecular compound.

Representing Compounds with Chemical Formulas

Types of Chemical Formulas

• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound. Example: The empirical formula for hydrogen peroxide (H2O2) is HO.

• Molecular Formula: Shows the actual number of atoms of each element in a molecule. Example: The molecular formula for glucose is C6H12O6.

• Structural Formula: Illustrates how atoms are connected in a molecule.

Key Point: Ionic compounds are typically represented by their empirical formulas, while molecular compounds may have molecular formulas that are multiples of their empirical formulas.

Ionic Compounds (Metal/Nonmetal)

Formation and Naming of Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between cations (positively charged ions, usually metals) and anions (negatively charged ions, usually nonmetals).

• Monatomic Ions: Ions consisting of a single atom (e.g., Na+, Cl-).

• Polyatomic Ions: Ions consisting of two or more atoms covalently bonded, carrying a net charge (e.g., NO3-, SO42-).

Naming Rules:

• Name the cation first, then the anion.

• For monatomic anions, change the ending to -ide (e.g., Cl- becomes chloride).

• For metals with variable charges (transition metals), indicate the charge with Roman numerals (e.g., FeCl3 is iron(III) chloride).

• For compounds containing polyatomic ions, use the name of the polyatomic ion as is (e.g., Ca(NO3)2 is calcium nitrate).

Common Polyatomic Ions

Formula	Name	Charge
NO3-	Nitrate	-1
SO42-	Sulfate	-2
CO32-	Carbonate	-2
PO43-	Phosphate	-3
OH-	Hydroxide	-1
NH4+	Ammonium	+1

Additional info: Students should memorize the names, formulas, and charges of common polyatomic ions.

Molecular Compounds (2 Nonmetals)

Naming Molecular Compounds

Molecular compounds are formed from two or more nonmetals. Their names use prefixes to indicate the number of each type of atom present.

• Name the first element in the formula.

• Name the second element with an -ide suffix.

• Use prefixes to indicate the number of atoms: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10)

• Drop the prefix mono- for the first element.

Example: CO2 is carbon dioxide; N2O5 is dinitrogen pentoxide.

Calculating Formula Masses

Formula Mass Calculation

The formula mass (also called molecular mass or molar mass) is the sum of the atomic masses of all atoms in a chemical formula.

• Multiply the atomic mass of each element by the number of times it appears in the formula.

• Add the results to obtain the total formula mass.

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

Example: For H2O: $2 \times 1.01\,\text{amu} + 16.00\,\text{amu} = 18.02\,\text{amu}$

Naming Binary Acids and Oxyacids

Acids: Binary and Oxyacids

Acids are molecular compounds that release hydrogen ions (H+) when dissolved in water. They are classified as binary acids or oxyacids based on their composition.

• Binary Acids: Composed of hydrogen and one nonmetal (e.g., HCl).

• Oxyacids: Composed of hydrogen, oxygen, and another element, usually containing a polyatomic ion (e.g., H2SO4).

Naming Binary Acids:

• Use the prefix hydro- followed by the base name of the nonmetal with an -ic ending, then add "acid".

• Example: HCl (aq) is hydrochloric acid.

Naming Oxyacids:

• If the polyatomic ion ends in -ate, change the ending to -ic and add "acid".

• If the polyatomic ion ends in -ite, change the ending to -ous and add "acid".

• Example: HNO3 (nitrate ion) becomes nitric acid; HNO2 (nitrite ion) becomes nitrous acid.

Summary Table: Compound Classification

Type	Constituents	Bond Type	Example
Atomic Element	Single atoms	None	Al (aluminum)
Molecular Element	Same element, multiple atoms	Covalent	Cl2 (chlorine)
Molecular Compound	Two or more nonmetals	Covalent	CO2 (carbon dioxide)
Ionic Compound	Metal and nonmetal	Ionic	NaCl (sodium chloride)
Acid	Hydrogen + nonmetal or polyatomic ion	Covalent (molecular)	HCl (hydrochloric acid)

Practice and Application

• Classify compounds as atomic element, molecular element, molecular compound, or ionic compound.

• Apply systematic naming rules for ionic and molecular compounds, including acids.

• Calculate formula masses for given chemical formulas.

Additional info: Real-world examples (e.g., table salt, sugar, cleaning agents) illustrate the importance of proper classification and nomenclature in chemistry.