Chapter 6: Ionic and Molecular Compounds – Structure, Bonding, and Properties

Ionic and Molecular Compounds

Introduction to Chemical Bonds

Chemical compounds are formed when atoms are joined by chemical bonds. There are two main types of bonds: ionic bonds, which involve the transfer of electrons, and covalent bonds, which involve the sharing of electrons. Understanding these bonds is essential for predicting compound properties, formulas, and names.

Ions and the Octet Rule

Formation of Ions

Atoms form ions to achieve a stable electron configuration, often resembling the nearest noble gas. This is known as the octet rule: atoms tend to lose, gain, or share electrons to obtain eight electrons in their valence shell (except for helium, which achieves stability with two electrons).

• Cation: A positively charged ion formed by the loss of one or more electrons (typically metals).

• Anion: A negatively charged ion formed by the gain of one or more electrons (typically nonmetals).

Predicting Ionic Charges

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

• Group 1A (1): Lose 1 electron to form 1+ cations (e.g., Na+).

• Group 2A (2): Lose 2 electrons to form 2+ cations (e.g., Mg2+).

• Group 3A (13): Lose 3 electrons to form 3+ cations (e.g., Al3+).

• Group 5A (15): Gain 3 electrons to form 3− anions (e.g., N3−).

• Group 6A (16): Gain 2 electrons to form 2− anions (e.g., O2−).

• Group 7A (17): Gain 1 electron to form 1− anions (e.g., Cl−).

Naming Ions

Monatomic Ions

• Cations: Named by the element name followed by "ion" (e.g., sodium ion, Mg2+ is magnesium ion).

• Anions: Named by the stem of the element name plus the suffix "-ide" (e.g., Cl− is chloride).

Metals with Variable Charges

Some transition metals can form more than one type of cation. Their charge is indicated by a Roman numeral in parentheses after the element name (e.g., Fe2+ is iron(II) ion, Fe3+ is iron(III) ion).

Ionic Compounds

Formation and Properties

Ionic compounds are formed from the electrostatic attraction between cations and anions. They are typically crystalline solids with high melting and boiling points, and they conduct electricity when dissolved in water.

Writing and Naming Ionic Compounds

• Binary Ionic Compounds: Consist of two elements (a metal and a nonmetal). Name the cation first, then the anion (e.g., NaCl is sodium chloride).

• Formulas: The total positive and negative charges must balance. Use the crisscross method to determine subscripts if necessary.

Polyatomic Ions

Polyatomic ions are groups of covalently bonded atoms with an overall charge. Common examples include sulfate (SO42−), nitrate (NO3−), and ammonium (NH4+).

Molecular Compounds: Covalent Bonding

Covalent Bonds and Molecules

A covalent bond is formed when two nonmetal atoms share electrons. A molecule is a discrete unit of atoms held together by covalent bonds. Molecular compounds exist as individual molecules, not as extended lattices like ionic compounds.

Naming Molecular Compounds

• Name the less electronegative element first, then the more electronegative element with the suffix "-ide".

• Use prefixes to indicate the number of each atom (mono-, di-, tri-, tetra-, etc.). The prefix "mono-" is often omitted for the first element.

• Example: CO2 is carbon dioxide; P4O6 is tetraphosphorus hexoxide.

Diatomic Molecules

Certain elements exist naturally as diatomic molecules (e.g., H2, N2, O2, F2, Cl2, Br2, I2).

Lewis Structures and Molecular Geometry

Lewis Structures

Lewis structures use dots to represent valence electrons and lines for shared pairs (bonds). They help visualize bonding and lone pairs, which are crucial for predicting molecular shape and polarity.

VSEPR Theory and Molecular Shapes

The Valence Shell Electron Pair Repulsion (VSEPR) Theory predicts the three-dimensional shape of molecules by arranging electron pairs as far apart as possible around the central atom.

• Linear: 2 charge clouds, 180° (e.g., CO2)

• Trigonal planar: 3 charge clouds, 120° (e.g., H2CO)

• Tetrahedral: 4 charge clouds, 109° (e.g., CH4)

• Trigonal pyramidal: 3 bonded atoms, 1 lone pair (e.g., NH3)

• Bent: 2 bonded atoms, 2 lone pairs (e.g., H2O)

Electronegativity and Bond Polarity

Electronegativity

Electronegativity (En) is a measure of an atom's ability to attract shared electrons in a bond. It increases across a period and decreases down a group.

Bond Polarity

The difference in electronegativity (ΔEn) between two atoms determines bond type:

• ΔEn = 0.0–0.4: Nonpolar covalent (electrons shared equally)

• ΔEn = 0.5–1.8: Polar covalent (electrons shared unequally)

• ΔEn = 1.9–3.3: Ionic (electrons transferred)

Polarity of Molecules

Determining Molecular Polarity

A molecule is polar if it contains polar bonds and the centers of partial positive (δ+) and partial negative (δ−) charge do not coincide. Molecular shape and bond polarity together determine overall molecular polarity.

Intermolecular Forces

Types of Intermolecular Forces

• Ionic bonds: Strongest, between ions (e.g., NaCl).

• Hydrogen bonds: Strong dipole-dipole attractions between H and N, O, or F.

• Dipole-dipole attractions: Between polar molecules.

• London dispersion forces: Weakest, due to temporary dipoles in all molecules, especially nonpolar ones.

Effect on Physical Properties

The strength of intermolecular forces influences melting and boiling points. Stronger forces result in higher melting and boiling points. For example, ionic compounds have much higher melting points than molecular compounds due to the strength of ionic bonds.

Summary Table: Types of Bonds and Forces

Type of Force	Example	Relative Strength
Ionic bond	NaCl	Strongest
Covalent bond	Cl2	Strong
Hydrogen bond	H2O	Moderate
Dipole-dipole	HCl	Weak
Dispersion forces	CH4	Weakest

Key Concepts to Master

• Explain and apply the octet rule.

• Distinguish between cations and anions; predict ion charges for representative elements.

• Name and write formulas for ions, ionic compounds, and molecular compounds.

• Draw Lewis structures and predict molecular shapes using VSEPR theory.

• Determine bond and molecular polarity using electronegativity differences and molecular geometry.

• Identify and compare intermolecular forces and their effects on physical properties.