Molecules and Compounds: Structure, Classification, and Nomenclature

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Chapter 5: Molecules and Compounds

Introduction to Compounds and Molecules

Chemical compounds are substances formed from two or more elements combined in fixed, definite proportions. The properties of compounds are often very different from those of the elements that compose them. For example, ordinary table sugar (sucrose) contains carbon, hydrogen, and oxygen, but its properties differ greatly from those of elemental carbon, hydrogen, or oxygen.

Compound: A substance composed of two or more elements in fixed, definite proportions.
Molecule: The smallest identifiable unit of a molecular compound, consisting of two or more atoms bonded together.
Example: Sucrose (table sugar) is a compound with the formula C12H22O11.

Law of Constant Composition

Definition and Application

The law of constant composition, also known as the law of definite proportions, states that all samples of a given compound have the same proportions of their constituent elements by mass.

Formulated by: Joseph Proust (1754–1826).
Example (Water): Decomposing 18.0 g of water yields 16.0 g of oxygen and 2.0 g of hydrogen, giving a mass ratio of .
Example (Ammonia): Decomposing 17.0 g of ammonia yields 14.0 g of nitrogen and 3.0 g of hydrogen, giving a mass ratio of .

Chemical Formulas

Types and Conventions

Chemical formulas represent the elements present in a compound and the relative number of atoms of each. Subscripts indicate the number of atoms; a subscript of 1 is omitted.

Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound.
Molecular Formula: Shows the actual number of atoms of each element in a molecule.
Structural Formula: Shows how atoms are connected using lines for chemical bonds.
Example: Hydrogen peroxide: Molecular formula is H2O2, empirical formula is HO.

Order of Elements in Formulas

Most metallic element is listed first (e.g., NaCl, not ClNa).
Among nonmetals, the more metal-like element (to the left or lower in the periodic table) is listed first (e.g., SO2, not O2S).

Polyatomic Ions in Formulas

Some compounds contain groups of atoms that act as a unit (polyatomic ions). Parentheses are used when more than one group is present.

Example: Mg(NO3)2 contains 1 Mg, 2 N, and 6 O atoms.

Classification of Elements and Compounds

Elements: Atomic vs. Molecular

Atomic Elements: Exist in nature with single atoms as their basic units (e.g., Hg, Na).
Molecular Elements: Exist as molecules composed of two or more atoms of the same element (e.g., O2, N2).

Elements That Occur as Diatomic Molecules

Name	Formula
Hydrogen	H2
Nitrogen	N2
Oxygen	O2
Fluorine	F2
Chlorine	Cl2
Bromine	Br2
Iodine	I2

Compounds: Molecular vs. Ionic

Molecular Compounds: Formed from two or more nonmetals; basic units are molecules (e.g., CO2).
Ionic Compounds: Contain cations (usually metals) and anions (usually nonmetals); basic units are formula units (e.g., NaCl).

Writing and Naming Ionic Compounds

Formulas for Ionic Compounds

Sum of charges of cations and anions must be zero.
Use the charge of one ion as the subscript for the other (criss-cross method).
Reduce subscripts to the smallest whole-number ratio.
Example: Al2O3 (aluminum oxide), MgO (magnesium oxide).

Naming Ionic Compounds

Type I: Metal forms only one type of cation (e.g., Na+, Ca2+).
Type II: Metal forms more than one type of cation (usually transition metals, e.g., Fe2+, Fe3+).

Common Anions and Their Names

Symbol	Base Name	Anion Name
F-	fluor-	fluoride
Cl-	chlor-	chloride
Br-	brom-	bromide
I-	iod-	iodide
O2-	ox-	oxide
S2-	sulf-	sulfide
N3-	nitr-	nitride

Metals That Form More Than One Ion

Symbol	Name	Older Name
Fe2+	iron(II)	ferrous
Fe3+	iron(III)	ferric
Cu+	copper(I)	cuprous
Cu2+	copper(II)	cupric
Sn2+	tin(II)	stannous
Sn4+	tin(IV)	stannic
Pb2+	lead(II)	plumbous
Pb4+	lead(IV)	plumbic

Naming Compounds with Polyatomic Ions

Use the name of the polyatomic ion whenever it occurs.
Example: KNO3 is potassium nitrate; Fe(OH)2 is iron(II) hydroxide; NH4NO3 is ammonium nitrate.

Oxyanions Naming

For two ions in a series: -ate (more O), -ite (less O).
For more than two: hypo- (least O), per- (most O).
Examples: NO3- (nitrate), NO2- (nitrite), ClO4- (perchlorate), ClO- (hypochlorite).

Naming Molecular Compounds

Rules and Prefixes

Formed from two or more nonmetals.
Use prefixes to indicate the number of each atom.
First element: prefix omitted if only one atom.
Second element: always use a prefix and end with -ide.

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

Example: CO2 is carbon dioxide; N2O is dinitrogen monoxide.

Naming Acids

Binary Acids

Composed of hydrogen and a nonmetal.
Name: hydro- + base name of nonmetal + -ic + acid.
Example: HCl(aq) is hydrochloric acid; H2S(aq) is hydrosulfuric acid.

Oxyacids

Contain hydrogen, a nonmetal, and oxygen (as part of a polyatomic ion).
If the polyatomic ion ends in -ate: base name + -ic + acid.
If the polyatomic ion ends in -ite: base name + -ous + acid.
Examples: HNO3 (nitric acid, from nitrate), HNO2 (nitrous acid, from nitrite), H2SO4 (sulfuric acid, from sulfate), H2SO3 (sulfurous acid, from sulfite).

Formula Mass

Definition and Calculation

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

Formula:
Example: For H2O: amu

Summary of Key Concepts

Compounds have constant composition; elements combine in fixed ratios.
Chemical formulas represent the composition and structure of compounds.
Nomenclature rules allow systematic naming of ionic, molecular compounds, and acids.
Formula mass is calculated by summing atomic masses according to the chemical formula.