2.6 Ionic and Molecular Compounds - Chemistry 2e | OpenStax

Learning Objectives

• Define ionic and molecular (covalent) compounds
• Predict the type of compound formed from elements based on their location within the periodic table
• Determine formulas for simple ionic compounds

Key Concepts

Ion Formation

• Ions: Electrically charged particles formed when atoms gain or lose electrons.
• Cations: Positively charged ions formed by metal atoms losing electrons.
• Anions: Negatively charged ions formed by nonmetal atoms gaining electrons.

Predicting Ionic Charges

• Main-group metals lose electrons to resemble preceding noble gas (e.g., alkali metals lose one electron to form 1+ ions).
• Nonmetals gain electrons to resemble the next noble gas (e.g., halogens gain one electron to form 1- ions).

Example: Sodium Ion Formation

• Sodium atom (Na) loses one electron, forming Na+ ion.
• Neutral sodium: 11 protons, 11 electrons. Na+ ion: 11 protons, 10 electrons.

Periodic Table Trends

• Group Trends: Moving left to right, metals form cations with a charge equal to group number.
• Transition Metals: Variable charges, not predictable by location (e.g., iron can be Fe2+ or Fe3+).
• Anion Formation: Moving right to left, elements form anions with a charge equal to the number of groups moved left from noble gases.

Polyatomic Ions

• Ions composed of multiple atoms that act as a single charged entity.
• Common Polyatomic Ions:
  • Ammonium: NH4+
  • Hydroxide: OH-
  • Sulfate: SO42-
• Memorize names, formulas, and charges as they are frequently used.

Ionic Bonds

• Formed by the electrostatic attraction between oppositely charged ions.
• Characteristic of compounds formed by metals and nonmetals.

Properties of Ionic Compounds

• Typically solid with high melting and boiling points (e.g., NaCl melts at 801°C).
• Conduct electricity when molten.

Molecular (Covalent) Compounds

• Formed by the sharing of electrons between nonmetals.
• Exist as gases, low-boiling liquids, or low-melting solids under normal conditions.

Examples of Ionic vs. Molecular Compounds

• Ionic: NaCl (sodium chloride), CaCl2 (calcium chloride).
• Covalent: H2O (water), CHCl3 (chloroform).

Predicting Formulas

• Ionic Compounds: Balance total positive and negative charges to ensure neutrality (e.g., Al2O3).
• Compounds with Polyatomic Ions: Consider polyatomic ions as single units, use parentheses to denote multiples (e.g., Ca(H2PO4)2).

Examples & Practice

1. Predicting Ionic Compounds:
   • Sodium cation + sulfide anion = Na2S.
2. Polyatomic Ion Formula:
   • Calcium dihydrogen phosphate = Ca(H2PO4)2.

Conclusion

Understanding ionic and molecular compounds involves knowing how ions form, predicting ion charges based on periodic trends, recognizing ionic and covalent bonds, and predicting compound formulas. Mastery of these concepts is foundational for further study in chemistry.