Nomenclature of Ionic Compounds: CHM 119 Lecture 4 Study Guide

Compounds & Molecules

Definitions and Classifications

Chemical compounds are pure substances composed of two or more elements in a fixed proportion, exhibiting a unique set of chemical properties. A molecule is a group of two or more atoms chemically bonded together. Elements can exist as atomic, diatomic, or polyatomic species.

• Atomic Elements: Elements that exist as single atoms, e.g., He, Ar, Ne, Kr (noble gases).

• Diatomic Elements: Elements that exist as molecules of two atoms, e.g., H2, O2, N2, F2, Cl2, Br2.

• Polyatomic Elements: Elements that exist as molecules of more than two atoms, e.g., P4, S8, C60.

Molecular Formula

Composition and Structure

The composition of a compound or molecule is fixed, and its molecular formula gives the identity (symbol) and number of each element present. The structural formula shows how atoms are attached (bonded) in the molecule.

• Example: $\mathrm{H_2SO_4}$ contains 2 hydrogen, 1 sulfur, and 4 oxygen atoms (total of 7 atoms).

• Molecular formula: $\mathrm{H_2O}$

• Structural formula: H—O—H

Naming Compounds

Purpose and Systems

It is important to clearly identify chemical compounds using names that provide information about their composition. Compounds are assigned names using systems developed by the International Union of Pure and Applied Chemistry (IUPAC). The process of assigning names for chemical compounds is called nomenclature.

Ionic Compounds

Composition and Charge Balance

Ionic compounds are made up of a cation (usually a metal) and an anion (usually a nonmetal). The net total charge of an ionic compound is always zero.

• Examples: NaCl, Al2S3, Cu(OH)2, NH4Cl, Ca(NO3)2

• Cations: Positively charged ions (e.g., Na+, Ca2+, Al3+)

• Anions: Negatively charged ions (e.g., Cl-, S2-, NO3-)

The sum of the charges in an ionic compound must equal zero:

• $1(+1) + 1(-1) = 0$

• $1(+2) + 2(-1) = 0$

• $2(+3) + 3(-2) = 0$

Ionic compounds tend to dissociate into their cations and anions in water, allowing the solution to conduct electricity (electrolytes).

Cation Charge (Type 1)

Type 1 Cations: Fixed Charge Metals

Type 1 cations have only one possible charge. For main group (Group 1A and 2A) metals, the group number corresponds to the positive charge.

Metal	Ion	Name	Group Number
Li	Li+	lithium	1A
Na	Na+	sodium	1A
K	K+	potassium	1A
Mg	Mg2+	magnesium	2A
Ca	Ca2+	calcium	2A
Al	Al3+	aluminum	3A
Ag	Ag+	silver	-

Some transition metals also form Type 1 cations.

Cation Charge (Type 2)

Type 2 Cations: Variable Charge Metals

Type 2 cations have several possible charges. These are typically transition metals and some main group metals.

Symbol	Ion	Name	Older Name
Cr	Cr2+	chromium(II)	chromous
Cr	Cr3+	chromium(III)	chromic
Fe	Fe2+	iron(II)	ferrous
Fe	Fe3+	iron(III)	ferric
Cu	Cu+	copper(I)	cuprous
Cu	Cu2+	copper(II)	cupric
Sn	Sn2+	tin(II)	stannous
Sn	Sn4+	tin(IV)	stannic
Pb	Pb2+	lead(II)	plumbous
Pb	Pb4+	lead(IV)	plumbic

Anion Charge

Common Anions and Naming Rules

Anions only have a single negative charge. The charge of a monatomic anion can be determined by the formula:

$\text{Anion Charge} = \text{Group Number} - 8$

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

• To name a monoatomic anion, use the root of the nonmetal name and add the suffix "-ide".

Binary Ionic Compounds

Definition and Naming Rules

Binary ionic compounds contain only two different elements. For compounds with a Type 1 cation (fixed charge), the charge of the metal is always the same and does not need to be specified in the name.

• Formula: (Name the cation) (Name the anion)

• Examples:

  • NaCl: Sodium chloride

  • K2S: Potassium sulfide

  • Al2O3: Aluminum oxide

  • Ca3P2: Calcium phosphide

Binary Ionic Compounds with Type 2 Cations

Naming Rules

For binary ionic compounds with a Type 2 cation (variable charge), the charge of the metal is specified using Roman numerals in parentheses.

• Formula: (Name the cation) (Cation charge in Roman numerals) (Name the anion)

• Examples:

  • FeCl2: Iron(II) chloride

  • CuO: Copper(II) oxide

  • Cr2S3: Chromium(III) sulfide

  • Pb3N4: Lead(IV) nitride

Polyatomic Ions in Ionic Compounds

Common Polyatomic Ions and Naming

Polyatomic cations and anions are treated as if they were monatomic ions in binary ionic compounds. Some common polyatomic ions must be memorized.

• Ammonium: NH4+

• Hydroxide: OH-

• Cyanide: CN-

• Examples:

  • NaOH: Sodium hydroxide

  • NH4Cl: Ammonium chloride

  • Fe(CN)2: Iron(II) cyanide

Additional info: The notes infer the importance of learning the charges and names of common ions, as well as the rules for naming compounds with both monatomic and polyatomic ions. The tables have been reconstructed for clarity and completeness.