Study Guide: Ionic and Molecular Compounds (Chapters 3-4)

Chapter 3: Ionic Compounds

Atoms, Cations, and Anions

Ions are charged particles formed when atoms gain or lose electrons. Cations are positively charged (typically metals), while anions are negatively charged (typically nonmetals).

• Cations: Formed by loss of electrons (e.g., Na+, Ca2+).

• Anions: Formed by gain of electrons (e.g., Cl-, O2-).

• Writing Formulas: The total positive charge must balance the total negative charge in an ionic compound.

Octet Rule and Ion Formation

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a full set of eight valence electrons, similar to noble gases. This drives the formation of ions.

• Application: Metals lose electrons to form cations; nonmetals gain electrons to form anions.

Periodic Trends in Ionization Energy and Electron Affinity

• Ionization Energy: The energy required to remove an electron from an atom. Increases across a period, decreases down a group.

• Electron Affinity: The energy change when an atom gains an electron. Generally becomes more negative across a period.

Ionic Bonding and Properties of Ionic Compounds

• Ionic Bond: The electrostatic attraction between oppositely charged ions.

• Properties: High melting and boiling points, crystalline solids, conduct electricity when molten or dissolved in water.

Writing Ionic Formulas and Names

• Combine cations and anions in ratios that result in a neutral compound.

• Use the suffix -ide for simple anions (e.g., chloride, oxide).

• Polyatomic ions have specific names and formulas (see Table 3.3 for common examples).

Polyatomic Ions

• Groups of atoms covalently bonded that carry a charge (e.g., SO42-, NO3-).

• Names and formulas must be memorized for common polyatomic ions.

Transition Metals and Variable Charges

• Some transition metals form more than one type of cation. Use Roman numerals to indicate the charge (e.g., Fe2+ is iron(II), Fe3+ is iron(III)).

Writing and Naming Ionic Compounds

• Combine the names of the cation and anion. For transition metals, include the Roman numeral.

• Example: FeCl2 is iron(II) chloride; Ca(NO3)2 is calcium nitrate.

Solubility of Ionic Compounds

• Solubility rules help predict whether an ionic compound will dissolve in water (see Table 3.4 for common rules).

Chapter 4: Molecular Compounds

Covalent Bonding and Molecules

Molecular compounds are formed when nonmetals share electrons to achieve the octet rule, resulting in covalent bonds.

• Covalent Bond: A shared pair of electrons between two atoms.

• Molecule: A group of atoms held together by covalent bonds (e.g., H2O, CO2).

Lewis Structures

• Visual representations of the arrangement of atoms and electrons in a molecule.

• Show all valence electrons as dots; shared pairs (bonds) are lines.

• Follow the octet rule for main group elements.

Multiple Bonds

• Atoms can share more than one pair of electrons: double bonds (O2), triple bonds (N2).

Electronegativity and Bond Polarity

• Electronegativity: The ability of an atom to attract shared electrons. Increases across a period, decreases down a group.

• Bond Polarity: Determined by the difference in electronegativity between atoms.

• Nonpolar covalent: equal sharing (e.g., Cl2); polar covalent: unequal sharing (e.g., H2O).

Molecular Shape (VSEPR Theory)

• Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the 3D shape of molecules based on electron pair repulsion.

• Common shapes: linear, bent, trigonal planar, tetrahedral.

Polarity of Molecules

• Depends on both bond polarity and molecular shape.

• Example: CO2 is nonpolar (linear shape), H2O is polar (bent shape).

Naming Molecular Compounds

• Use prefixes to indicate the number of each atom (mono-, di-, tri-, etc.).

• Example: CO2 is carbon dioxide; N2O4 is dinitrogen tetroxide.

Summary Table: Common Prefixes for Molecular Compounds

Number	Prefix
1	mono-
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-

Practice Problems

• Practice writing formulas and names for both ionic and molecular compounds.

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

Additional info: For more details on polyatomic ions, solubility rules, and VSEPR shapes, refer to textbook tables and figures as indicated in the original notes.