Ionic and Molecular Compounds: Structure, Formation, and Nomenclature

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Ionic and Molecular Compounds

Introduction

Ionic and molecular compounds are fundamental to understanding chemical bonding and the properties of substances. This section explores how atoms form ions, the nature of ionic and covalent bonds, and the rules for naming and writing formulas for these compounds.

6.1 Ions: Transfer of Electrons

Formation of Ions

Atoms form ions by gaining or losing electrons to achieve a stable electron configuration, typically that of the nearest noble gas (the octet rule). Metals tend to lose electrons to form cations (positively charged ions), while nonmetals gain electrons to form anions (negatively charged ions).

Ionic bonds are the strong attractive forces between positive and negative ions.
Metals (Groups 1A, 2A, 3A) lose electrons; nonmetals (Groups 5A, 6A, 7A) gain electrons.

Diagram showing transfer of electrons in ionic and covalent bonds

Types of Particles and Bonds in Compounds

The type of bonding and particles present in a compound determine its properties.

Type	Particles	Bonds	Examples
Ionic Compounds	Ions	Ionic	Na+ Cl- ions
Molecular Compounds	Molecules	Covalent	H2O, C3H8

Table of types of particles and bonds in compounds

Formation of Positive Ions (Cations)

Metals lose electrons to form cations with a positive charge.
Example: Sodium (Na) loses one electron to form Na+.

Sodium atom losing one electron to form Na+ ion

Example: Magnesium (Mg) loses two electrons to form Mg2+.

Magnesium atom losing two electrons to form Mg2+ ion

Formation of Negative Ions (Anions)

Nonmetals gain electrons to form anions with a negative charge.
Example: Chlorine (Cl) gains one electron to form Cl-.

Chlorine atom gaining one electron to form Cl- ion

Ionic Charges from Group Numbers

The periodic table group number helps predict the charge of ions formed by representative elements.

Noble Gas	1A (1)	2A (2)	3A (13)	5A (15)	6A (16)	7A (17)
He	Li+
Ne	Na+	Mg2+	Al3+
Ar	K+	Ca2+		P3-	S2-	Cl-
Kr	Rb+	Sr2+			Se2-	Br-
Xe	Cs+	Ba2+				I-

Table of monatomic ions and their nearest noble gases

6.2 Ionic Compounds

Formation and Properties of Ionic Compounds

Ionic compounds are formed when metals react with nonmetals, resulting in the transfer of electrons and the formation of a crystal lattice of alternating positive and negative ions.

Example: Sodium (Na) reacts with chlorine (Cl) to form sodium chloride (NaCl), or table salt.
Ionic compounds have high melting points and are solid at room temperature.

Sodium chloride crystal structure

Writing Formulas for Ionic Compounds

The chemical formula for an ionic compound reflects the lowest whole-number ratio of ions that results in a neutral compound (total positive charge equals total negative charge).

Subscripts are used to indicate the number of each ion needed for charge balance.

Diagram showing charge balance in NaCl formation

Example: Mg2+ and Cl- combine to form MgCl2.

Diagram showing charge balance in MgCl2 formation

Example: Na+ and S2- combine to form Na2S.

Diagram showing charge balance in Na2S formation

6.3 Naming and Writing Ionic Formulas

Naming Ionic Compounds

The name of an ionic compound consists of the name of the metal (cation) followed by the name of the nonmetal (anion) with its ending changed to -ide.

Example: NaCl is named sodium chloride.
Example: CaO is named calcium oxide.

Core chemistry skill: Naming ionic compounds

Formulas and Names for Common Ionic Compounds

	Br-	S2-	N3-
Na+	NaBr sodium bromide	Na2S sodium sulfide	Na3N sodium nitride
Al3+	AlBr3 aluminum bromide	Al2S3 aluminum sulfide	AlN aluminum nitride

Table of formulas and names for common ionic compounds

Metals with Variable Charges

Many transition metals can form more than one type of positive ion. The charge is specified using a Roman numeral in parentheses after the metal's name.