Ch.3 Molecules and Compounds: Study Notes

Ch.3 Molecules and Compounds

Introduction to Compounds

Compounds are a type of pure substance composed of two or more different elements in fixed, definite proportions. They are chemically combined and have properties distinct from their constituent elements.

• Pure substance: A material with a constant composition.

• Compound: A pure substance made of two or more elements chemically bonded in a fixed ratio.

• Example: Water (H2O) always contains two hydrogen atoms for every one oxygen atom.

Types of Chemical Bonds

Atoms in compounds are held together by chemical bonds, which result from attractions between charged particles (electrons and protons).

• Ionic bonds: Formed between metals and nonmetals, involving the transfer of electrons from the metal (forming a positively charged cation) to the nonmetal (forming a negatively charged anion).

• Covalent bonds: Formed between nonmetals, involving the sharing of electrons between atoms.

• Electrostatic forces: Oppositely charged ions attract each other, resulting in the formation of ionic compounds with a three-dimensional crystal lattice in the solid state.

• Molecule: A group of atoms covalently bonded together, forming the smallest unit of a molecular compound.

Representing Compounds

Compounds are represented by chemical formulas, which indicate the elements present and the relative number of atoms or ions of each element.

• Empirical formula: Shows the simplest whole-number ratio of atoms in a compound.

• Molecular formula: Shows the actual number of atoms of each element in a molecule.

• Structural formula: Shows how atoms are bonded and arranged in space.

• Example: Hydrogen peroxide has a molecular formula of H2O2 and an empirical formula of HO.

Classification of Elements and Compounds

Elements can exist as individual atoms or as molecules. Some elements are diatomic, meaning they exist as molecules composed of two atoms.

• Diatomic elements: H2, N2, O2, F2, Cl2, Br2, I2 (these 7 should be memorized).

• Molecular compounds: Composed of molecules (e.g., H2O, CO2).

• Ionic compounds: Composed of ions arranged in a crystal lattice (e.g., NaCl).

Ionic Compounds

Ionic compounds are represented by their formula unit, the smallest electrically neutral collection of ions. They always contain both cations (positively charged ions) and anions (negatively charged ions).

• Formula unit: The simplest ratio of ions in an ionic compound.

• Polyatomic ion: An ion composed of two or more covalently bonded atoms with an overall charge (e.g., ClO−, the hypochlorite ion).

Naming Ionic Compounds

The chemical formula for an ionic compound reflects the smallest whole-number ratio of cations to anions that results in a neutral compound.

• To name ionic compounds:

  1. Identify the cation and anion.

  2. Balance the charges so the total positive and negative charges are equal.

  3. Use the name of the cation (metal or polyatomic ion) followed by the name of the anion (nonmetal with -ide ending or polyatomic ion name).

• Metals with invariant charge: Some metals always have the same charge (e.g., Na+, Ca2+).

• Metals with variable charge: Some metals can have different charges (e.g., Fe2+, Fe3+), which must be indicated in the compound name.

• Memorization: It is important to memorize the charges of common metals and polyatomic ions.

Common Polyatomic Ions	Formula	Charge
Ammonium	NH4+	+1
Nitrate	NO3−	−1
Sulfate	SO42−	−2
Phosphate	PO43−	−3
Hydroxide	OH−	−1
Carbonate	CO32−	−2
Hypochlorite	ClO−	−1

Practice Problems

• Write the empirical formula for the following molecular formulas:

  • C5H12: Empirical formula is C5H12 (already simplest ratio).

  • Hg2Cl2: Empirical formula is HgCl.

  • C2H4O2: Empirical formula is CH2O.

• Classify substances as atomic element, molecular element, molecular compound, or ionic compound:

  • Fluorine: Molecular element (F2).

  • N2O: Molecular compound.

  • Silver: Atomic element (Ag).

  • K2O: Ionic compound.

  • Fe2O3: Ionic compound.

Key Equations and Concepts

• Empirical formula calculation: $\text{Empirical formula} = \frac{\text{Moles of each element}}{\text{Smallest number of moles}}$

• Charge balance in ionic compounds: $\sum \text{(charges of cations)} + \sum \text{(charges of anions)} = 0$

Summary Table: Types of Substances

Type	Example	Key Features
Atomic Element	Na, He	Single atoms as basic units
Molecular Element	O2, N2	Diatomic or polyatomic molecules
Molecular Compound	H2O, CO2	Molecules as basic units, covalent bonds
Ionic Compound	NaCl, CaF2	Formula units, ionic bonds

Additional info: Students should be familiar with the periodic table locations of metals, nonmetals, and the common polyatomic ions for effective compound naming and formula writing.