Study Guide: Molecules, Compounds, and Chemical Bonding

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, retaining its chemical properties.

• Molecular Element: An element whose particles are multi-atom molecules (e.g., O2, N2).

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

Identifying Elements in a Formula

To determine which elements are present in a chemical formula, analyze the symbols:

• Each capital letter represents a new element (e.g., NaCl contains sodium and chlorine).

• Common diatomic elements: Br2, I2, N2, Cl2, H2, O2, F2.

Predicting Compound Type from Formula

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

• Molecular Compound: Composed of nonmetals only.

• Ionic Compound: Composed of metals and nonmetals.

Example: NaCl is ionic; CO2 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 ionic compounds involves identifying the cation and anion and using appropriate rules:

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

• Use Roman numerals for metals with variable charges (e.g., iron(III) chloride).

• Polyatomic ions retain their names in compounds (e.g., sodium sulfate).

Naming Molecular Compounds

Molecular compounds are named using prefixes to indicate the number of atoms:

• Mono-, di-, tri-, tetra-, penta-, etc. (e.g., carbon dioxide, dinitrogen tetroxide).

Writing Chemical Formulas

To write the formula for a compound:

• Combine the symbols for the elements or polyatomic ions.

• Balance charges for ionic compounds.

• Use prefixes for molecular compounds.

Chapter 10: Chemical Bonding & Molecular Structure

Types of Chemical Bonds

Chemical bonds hold atoms together in compounds. The main types are:

• Ionic Bonds: Transfer of electrons from metal to nonmetal.

• Covalent Bonds: Sharing of electrons between nonmetals.

• Metallic Bonds: Delocalized electrons among metal atoms.

Lewis Structures

Lewis structures represent the arrangement of electrons in molecules:

• Show all valence electrons as dots or lines.

• Single, double, or triple bonds are represented by one, two, or three lines.

• Include lone pairs on atoms.

VSEPR Theory and Molecular Shapes

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes:

• Electron pairs (bonding and lone pairs) repel each other, determining geometry.

• Common shapes include:

  • Linear

  • Trigonal planar

  • Tetrahedral

  • Trigonal bipyramidal

  • Octahedral

Polarity of Molecules

Molecular polarity depends on bond polarity and molecular shape:

• Polar molecules have an uneven distribution of charge.

• Nonpolar molecules have an even distribution of charge.

• Use electronegativity differences and geometry to determine polarity.

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

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; present in double and triple bonds.

Counting Bonds:

• Single bond: 1 σ

• Double bond: 1 σ + 1 π

• Triple bond: 1 σ + 2 π

Formulas and Names of Polyatomic Ions

Polyatomic ions are charged species composed of two or more atoms covalently bonded. Their names and formulas must be memorized for chemical nomenclature.

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

Additional info:

• To predict molecular shape, use the VSEPR model and count electron domains around the central atom.

• For ionic compounds, the formula unit represents the simplest ratio of ions.

• For molecular compounds, the formula shows the exact number of atoms of each element.