Study Guide: Chapter 5

Chapter 5: Molecules and Compounds

Distinguishing Atoms, Molecular Elements, and Compounds

Understanding the differences between atoms, molecular elements, and compounds is fundamental in chemistry.

• Atom: The smallest unit of an element that retains its chemical properties.

• Molecular Element: An element that exists as molecules composed of two or more atoms of the same element (e.g., O2, N2).

• Compound: A substance formed from two or more different elements chemically bonded together (e.g., H2O, CO2).

Example: Oxygen gas (O2) is a molecular element, while water (H2O) is a compound.

Identifying Elements in a Formula

To determine whether an element is found in a molecule, examine the chemical formula for its symbol.

• For example, in H2SO4, the elements present are H, S, and O.

Predicting Compound Type from Formula

Compounds can be classified as ionic or molecular based on their composition.

• Ionic Compound: Contains a metal and a nonmetal or a polyatomic ion (e.g., NaCl, NH4NO3).

• Molecular Compound: Contains only nonmetals (e.g., CO2, H2O).

Example: Na2SO4 is ionic; CH4 is molecular.

Properties of Ionic and Molecular Compounds

• Ionic Compounds: High melting points, conduct electricity when dissolved in water.

• Molecular Compounds: Lower melting points, do not conduct electricity in water.

Naming Ionic Compounds

Naming conventions depend on the type of ions present.

• For transition and post-transition metals, determine the charge of the ion from the name or formula.

• Use Roman numerals to indicate the charge (e.g., iron(III) chloride).

Naming Molecular Compounds

• Use prefixes to indicate the number of atoms (e.g., dinitrogen tetroxide).

• Write the chemical formula from the compound name and vice versa.

Chapter 10: Chemical Bonding & Molecular Structure

Types of Chemical Bonds

Chemical bonds hold atoms together in molecules and compounds.

• Ionic Bonds: Formed by the transfer of electrons from a metal to a nonmetal.

• Covalent Bonds: Formed by the sharing of electrons between nonmetals.

Drawing and Interpreting Lewis Structures

Lewis structures represent the arrangement of electrons in molecules.

• Show all valence electrons as dots.

• Indicate bonding pairs (lines) and lone pairs (dots).

• Apply the octet rule for main group elements.

Example: The Lewis structure for water (H2O) shows two bonding pairs and two lone pairs on oxygen.

Predicting Molecular Geometry (VSEPR Theory)

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes based on electron pair repulsion.

• Count bonding pairs and lone pairs around the central atom.

• Electron geometries include:

  • Linear

  • Trigonal planar

  • Tetrahedral

  • Trigonal bipyramidal

  • Octahedral

Example: CO2 is linear; CH4 is tetrahedral.

Polarity of Molecules

Molecular polarity depends on bond polarity and molecular geometry.

• Use electronegativity differences to determine bond polarity.

• Use molecular geometry to determine if dipoles cancel (nonpolar) or reinforce (polar).

Example: CO2 is nonpolar; H2O is polar.

Sigma and Pi Bonds

Covalent bonds can be classified as sigma (σ) or pi (π) bonds.

• Sigma (σ) Bond: Formed by head-on overlap of orbitals; every single bond is a sigma bond.

• Pi (π) Bond: Formed by side-on overlap of p orbitals; present in double and triple bonds.

Counting Bonds: In a double bond, there is one sigma and one pi bond; in a triple bond, one sigma and two pi bonds.

Formulas and Names of Polyatomic Ions

Common Polyatomic Ions

Polyatomic ions are charged species composed of two or more atoms covalently bonded.

Formula	Name
NH4+	ammonium
OH-	hydroxide
CO32-	carbonate
HCO3-	bicarbonate (hydrogen carbonate)
NO3-	nitrate
NO2-	nitrite
SO42-	sulfate
SO32-	sulfite
ClO4-	perchlorate
ClO3-	chlorate
ClO2-	chlorite
ClO-	hypochlorite
CN-	cyanide
PO43-	phosphate
HPO42-	hydrogen phosphate
H2PO4-	dihydrogen phosphate
SO42-	sulfate

Application: Recognizing these ions is essential for naming ionic compounds and predicting their properties.

Additional info:

• VSEPR stands for Valence Shell Electron Pair Repulsion theory.

• Electronegativity is a measure of an atom's ability to attract electrons in a bond.

• Lewis structures help visualize electron arrangement and predict reactivity.