Ionic and Molecular Compounds

Introduction

This study guide covers the fundamental concepts of ionic and molecular compounds, including the types of chemical bonds, how to determine ionic charges using the periodic table, and the rules for naming and writing formulas for both simple and polyatomic ionic compounds.

Types of Chemical Bonds

Ionic and Covalent Bonds

• Ionic Bond: A chemical bond formed when valence electrons are transferred from a metal atom to a nonmetal atom, resulting in the formation of ions.

• Covalent Bond: A chemical bond formed when two nonmetal atoms share valence electrons, resulting in the formation of molecules.

Octet Rule: Atoms tend to lose, gain, or share electrons to achieve a stable electron arrangement with eight electrons in their outer shell (except for hydrogen and helium).

Ionic vs. Molecular Compounds

Comparison of Properties

Ionic and molecular compounds differ in the types of particles they contain, the nature of their bonds, and their physical properties.

• Ionic compounds are typically solid at room temperature and have high melting points due to strong electrostatic attractions between ions.

• Molecular compounds can be gases, liquids, or solids and generally have lower melting points.

Ions and Ionic Charges

Formation of Ions

• Cations: Positively charged ions formed when atoms (usually metals) lose electrons.

• Anions: Negatively charged ions formed when atoms (usually nonmetals) gain electrons.

Groups 1, 2, and 13 of the periodic table readily lose their valence electrons to form cations. Groups 15, 16, and 17 readily gain electrons to form anions. The name of an anion is modified to end with "-ide" (e.g., chlorine becomes chloride).

Determining Ionic Charges from the Periodic Table

• Group 1A (1): loses 1 electron → +1 charge

• Group 2A (2): loses 2 electrons → +2 charge

• Group 3A (13): loses 3 electrons → +3 charge

• Group 5A (15): gains 3 electrons → -3 charge

• Group 6A (16): gains 2 electrons → -2 charge

• Group 7A (17): gains 1 electron → -1 charge

Properties of Ionic Compounds

Formation and Physical Properties

• Formation: Occurs when one or more electrons are transferred from metals to nonmetals, creating an ionic bond between the resulting ions.

• Properties:

  • Strong attraction between oppositely charged ions

  • High melting points

  • Solid at room temperature

Chemical Formulas of Ionic Compounds

Writing Formulas

• The chemical formula represents the symbols and subscripts in the lowest whole-number ratio of atoms or ions.

• Subscripts are used to indicate when more than one ion is needed to balance the charge.

• The total positive and negative charges must add up to zero.

Example: Magnesium ion (Mg2+) combines with two chloride ions (Cl-) to form MgCl2.

Naming Ionic Compounds

Rules for Naming

• The name consists of two elements: the metal (cation) and the nonmetal (anion).

• The cation name does not change.

• The anion name changes to end with "-ide".

• Subscripts are not included in the name.

Example: MgCl2 is named magnesium chloride.

Transition Metals with Variable Charge

• Many transition metals can form more than one type of positive ion (variable charge).

• The charge is indicated by a Roman numeral in parentheses after the metal name.

• Steps:

  1. Determine the charge of the cation from the anion.

  2. Name the cation with its elemental name and the Roman numeral for its charge.

  3. Name the anion with the "-ide" suffix.

Example: MnF2 is named manganese(II) fluoride.

Polyatomic Ions

Definition and Naming

• Polyatomic ion: A group of covalently bonded atoms that has an overall ionic charge.

• Most polyatomic ions are anions (negatively charged), except for ammonium (NH4+).

• Names often end in "-ate" or "-ite" depending on the number of oxygen atoms.

• "Per-" and "hypo-" prefixes indicate more or fewer oxygen atoms, respectively.

Example: ClO4- is perchlorate, ClO3- is chlorate, ClO2- is chlorite, and ClO- is hypochlorite.

Writing Formulas for Compounds with Polyatomic Ions

Rules and Examples

• Combine the cation and polyatomic anion so that the total charge is zero.

• Use parentheses around the polyatomic ion if more than one is needed.

• Write the cation first, followed by the polyatomic ion.

Example: Magnesium nitrate: Mg2+ and NO3- combine to form Mg(NO3)2.

Summary Table: Common Polyatomic Ions

Key Equations and Notation

• General formula for ionic compounds: where and are chosen to balance the charges.

• Example:

Practice and Application

• Be able to determine the charge of ions based on their group number in the periodic table.

• Write correct formulas for ionic compounds, including those with polyatomic ions.

• Name ionic compounds, including those with transition metals and polyatomic ions, using the appropriate rules.

Additional info: For further practice, create flashcards for common polyatomic ions and their charges, and practice naming and writing formulas for a variety of ionic compounds.