Chapter 5: Molecules and Compounds – Study Notes for Introductory Chemistry

Molecules and Compounds

Introduction to Compounds

Compounds are substances composed of two or more elements chemically combined in fixed, definite proportions. The properties of compounds are generally very different from those of the elements that compose them. For example, sucrose (table sugar) is made of carbon, hydrogen, and oxygen, but its properties differ greatly from those of its constituent elements.

Properties of Compounds vs. Elements

When elements combine to form compounds, the resulting substance often has properties distinct from the individual elements. For instance, sodium is a reactive metal, and chlorine is a poisonous yellow gas, but together they form sodium chloride (table salt), which is safe to eat.

Law of Constant Composition

Definition and Explanation

The law of constant composition (Joseph Proust) states that all samples of a given compound have the same proportions of their constituent elements. This means that compounds have a fixed, definite composition, unlike mixtures, which can have variable proportions.

Mixtures vs. Compounds

• Mixture: The relative amounts of components can vary (e.g., a balloon filled with hydrogen and oxygen gases).

• Compound: The elements combine in a fixed ratio (e.g., water always has two hydrogen atoms for every one oxygen atom).

Examples of Constant Composition

• Water (H2O): Decomposition of 18.0 g of water yields 16.0 g of oxygen and 2.0 g of hydrogen. The mass ratio is:

$\text{mass ratio} = \frac{16.0\ \text{g O}}{2.0\ \text{g H}} = 8.0$ or $8.0:1$

• Ammonia (NH3): Decomposition of 17.0 g of ammonia yields 14.0 g of nitrogen and 3.0 g of hydrogen. The mass ratio is:

$\text{mass ratio} = \frac{14.0\ \text{g N}}{3.0\ \text{g H}} = 4.7$ or $4.7:1$

Chemical Formulas

How to Represent Compounds

A chemical formula indicates the elements present in a compound and the relative number of atoms of each. Subscripts show the number of each atom; a subscript of 1 is omitted by convention.

• Examples: H2O (water), NaCl (table salt), CO2 (carbon dioxide), C12H22O11 (sucrose).

Importance of Subscripts

Changing a subscript changes the compound entirely. For example, CO is carbon monoxide, while CO2 is carbon dioxide—two different substances with different properties.

Order of Elements in Formulas

Elements are listed in order of increasing nonmetallic character. Metals are listed first; among nonmetals, those to the left and lower in the periodic table are listed before those to the right and higher.

Polyatomic Ions in Formulas

Some compounds contain polyatomic ions, groups of atoms that act as a unit and carry a charge. Parentheses are used when more than one polyatomic ion is present.

For example, in Mg(NO3)2:

• Mg: 1 atom

• N: 2 atoms (1 per NO3 × 2)

• O: 6 atoms (3 per NO3 × 2)

Types of Chemical Formulas and Models

Empirical, Molecular, and Structural Formulas

• Empirical formula: Shows the simplest whole-number ratio of atoms (e.g., HO for hydrogen peroxide).

• Molecular formula: Shows the actual number of atoms (e.g., H2O2 for hydrogen peroxide).

• Structural formula: Shows how atoms are connected.

Molecular models (ball-and-stick and space-filling) visually represent the three-dimensional arrangement of atoms.

Connecting Macroscopic and Molecular Worlds

Chemists use symbolic representations to connect what we see (macroscopic) with the atomic and molecular world. For example, water can be represented as a liquid, as molecules, and as a chemical formula.

Classification of Elements and Compounds

Elements: Atomic vs. Molecular

• Atomic elements: Exist as single atoms (e.g., mercury, Hg).

• Molecular elements: Exist as molecules, usually diatomic (e.g., Cl2, O2).

Diatomic Elements

Certain elements naturally exist as diatomic molecules. These include hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine.

Compounds: Molecular vs. Ionic

• Molecular compounds: Formed from two or more nonmetals; basic units are molecules (e.g., CO2).

• Ionic compounds: Contain cations (usually metals) and anions (usually nonmetals); basic units are formula units (e.g., NaCl).

Ionic Compounds

Formation and Structure

Ionic compounds form when metals transfer electrons to nonmetals, creating positive (cation) and negative (anion) ions that attract each other. The formula unit represents the simplest ratio of ions in the compound.

Writing Formulas for Ionic Compounds

1. Write the symbol and charge for each ion.

2. Cross the magnitude of each ion's charge to become the subscript for the other ion.

3. Reduce subscripts to the smallest whole-number ratio.

4. Check that the total positive and negative charges balance to zero.

Polyatomic Ions

Polyatomic ions are groups of atoms with an overall charge. Common examples include nitrate (NO3−), sulfate (SO42−), and ammonium (NH4+).

Nomenclature: Naming Compounds

Naming Ionic Compounds

To name an ionic compound, first identify it as containing a metal and one or more nonmetals. The naming depends on whether the metal forms only one type of ion (Type I) or more than one (Type II).

Type I: Metal with Invariant Charge

• Name = name of cation (metal) + base name of anion (nonmetal) + -ide

• Example: NaCl is sodium chloride.

Type II: Metal with Variable Charge

• Name = name of cation (metal) + (charge in Roman numerals) + base name of anion + -ide

• Example: FeCl3 is iron(III) chloride.

Naming Compounds with Polyatomic Ions

• Use the name of the polyatomic ion whenever it occurs in the formula.

• Example: KNO3 is potassium nitrate; Fe(OH)2 is iron(II) hydroxide.

Naming Oxyanions and Related Compounds

• Oxyanions are polyatomic ions containing oxygen.

• -ate: More oxygen atoms (e.g., nitrate, NO3−); -ite: Fewer oxygen atoms (e.g., nitrite, NO2−).

• Prefixes: hypo- (less than), per- (more than) for series with more than two ions.

Polyatomic Ions in Everyday Chemistry

Examples include sodium hypochlorite (bleach), sodium bicarbonate (baking soda), calcium carbonate (antacids), and sodium nitrite (food preservative).

Naming Molecular Compounds

Rules for Naming

• Formed from two or more nonmetals.

• Prefixes indicate the number of each atom (mono-, di-, tri-, tetra-, etc.).

• The first element is the more metal-like one; the prefix mono- is omitted for the first element if only one atom is present.

Naming Acids

Classification of Acids

• Binary acids: Contain hydrogen and one nonmetal.

• Oxyacids: Contain hydrogen, a nonmetal, and oxygen (usually as part of a polyatomic ion).

Naming Binary Acids

• Name = hydro + base name of nonmetal + -ic + acid

• Example: HCl(aq) is hydrochloric acid.

Naming Oxyacids

• If the polyatomic ion ends in -ate, the acid name ends in -ic acid (e.g., HNO3 is nitric acid).

• If the polyatomic ion ends in -ite, the acid name ends in -ous acid (e.g., HNO2 is nitrous acid).

Nomenclature Flowchart and Examples

A flowchart can help determine the correct naming path for any compound, whether ionic, molecular, or acidic.

Formula Mass

Definition and Calculation

The formula mass of a compound is the sum of the atomic masses of all atoms in its chemical formula. It is calculated as follows:

$\text{formula mass} = (\#\ \text{atoms of 1st element} \times \text{atomic mass of 1st element}) + (\#\ \text{atoms of 2nd element} \times \text{atomic mass of 2nd element}) + \ldots$

Summary of Key Concepts

• Compounds have constant composition; elements combine in fixed ratios.

• Chemical formulas indicate the elements and their proportions in a compound.

• Nomenclature rules allow systematic naming of ionic, molecular compounds, and acids.

• Formula mass is the sum of atomic masses in a compound's formula.