Ionic and Molecular Compounds: Structure, Naming, and Formulas

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Ionic and Molecular Compounds

Types of Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds. The two main types are ionic bonds and covalent bonds. Ionic bonds involve the transfer of electrons from metals to nonmetals, forming ions, while covalent bonds involve the sharing of electrons between nonmetals to form molecules.

Ionic Compounds: Composed of positive and negative ions held together by electrostatic forces.
Molecular Compounds: Consist of molecules formed by atoms sharing electrons through covalent bonds.

Comparison of ionic and molecular compounds: particles, bonds, and examples

The Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons, similar to noble gases. This rule explains the formation of both ionic and covalent bonds.

Ions: Formation and Properties

Formation of Ions

Ions are atoms or groups of atoms with a net electrical charge due to the loss or gain of electrons. Metals lose electrons to form cations (positive ions), while nonmetals gain electrons to form anions (negative ions).

Cations: Formed by metals losing valence electrons.
Anions: Formed by nonmetals gaining valence electrons.

Sodium atom losing one electron to form a sodium ion Magnesium atom losing two electrons to form a magnesium ion Chlorine atom gaining one electron to form a chloride ion

Determining Ionic Charges from the Periodic Table

The charge of an ion formed by a representative element can often be predicted from its group number:

Group 1A: +1
Group 2A: +2
Group 3A: +3
Group 5A: -3
Group 6A: -2
Group 7A: -1

Table of metals and nonmetals showing how many electrons are lost or gained to form ions

Writing and Naming Ionic Compounds

Formation of Ionic Compounds

Ionic compounds form when electrons are transferred from metals to nonmetals, resulting in a strong electrostatic attraction between oppositely charged ions. The chemical formula represents the lowest whole-number ratio of ions that results in a neutral compound.

Always write the cation first, followed by the anion.
Use subscripts to indicate the number of each ion needed to balance the charges.

Formation of magnesium chloride from magnesium and chlorine atoms Formation of sodium chloride from sodium and chlorine atoms Formation of sodium sulfide from sodium and sulfur atoms

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." Subscripts are not included in the name.

Example: NaCl is named sodium chloride.
Example: MgCl2 is named magnesium chloride.

Table of ionic compounds, their ions, and names

Metals with Variable Charges (Transition Metals)

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.

Example: FeCl2 is iron(II) chloride; FeCl3 is iron(III) chloride.
Exceptions: Zn2+, Cd2+, and Ag+ have only one common charge.

Table of transition metals with possible ions and their names

Determining the Charge of Transition Metals in Compounds

To determine the charge of a transition metal in an ionic compound:

Calculate the total negative charge from the anion(s).
The total positive charge must balance the negative charge.
Assign the Roman numeral to the metal based on its charge.

Example: In MnF2, each F is -1, so Mn must be +2. The name is manganese(II) fluoride.

Periodic Table of Ions

The periodic table can be used to predict the charges of ions formed by main group elements and to identify common ions of transition metals.

Periodic table highlighting common ions for metals and nonmetals

Writing Formulas from Names

To write the formula for an ionic compound from its name:

Identify the cation and its charge.
Identify the anion and its charge.
Balance the charges so the total is zero.
Write the formula with the cation first, using subscripts as needed.

Polyatomic Ions

Definition and Common Polyatomic Ions

Polyatomic ions are groups of covalently bonded atoms that carry an overall charge. Most are anions, except for ammonium (NH4+).

Names ending in "-ate" have more oxygen atoms than those ending in "-ite."
Prefixes "per-" and "hypo-" indicate more or fewer oxygen atoms, respectively.

Examples:

ClO4-: Perchlorate
ClO3-: Chlorate
ClO2-: Chlorite
ClO-: Hypochlorite

Table of common polyatomic ions, their formulas, and names

Writing Formulas for Compounds with Polyatomic Ions

When writing formulas for compounds containing polyatomic ions, balance the total positive and negative charges. Use parentheses around the polyatomic ion if more than one is needed.

Example: Magnesium nitrate is Mg(NO3)2.

Formation of magnesium nitrate with parentheses for polyatomic ions

Naming Compounds with Polyatomic Ions

Follow the same rules as for simple ionic compounds: name the cation first, then the polyatomic ion. Recognition of polyatomic ions is essential for correct naming.

Example: Na2SO4 is sodium sulfate.
Example: FePO4 is iron(III) phosphate.

Examples of naming compounds with polyatomic ions

Summary Table: Key Concepts in Ionic and Molecular Compounds

Concept	Description
Ionic Bond	Transfer of electrons from metal to nonmetal, forming ions
Covalent Bond	Sharing of electrons between nonmetals
Cation	Positively charged ion (metal loses electrons)
Anion	Negatively charged ion (nonmetal gains electrons)
Polyatomic Ion	Group of covalently bonded atoms with an overall charge
Formula Unit	Lowest whole-number ratio of ions in an ionic compound

Additional info: For further mastery, students should practice writing names and formulas for both simple and polyatomic ionic compounds, and memorize the most common polyatomic ions and their charges.