Chapter 5: Molecules and Compounds – Study Notes

Chapter 5: Molecules and Compounds

Introduction

This chapter explores the nature of molecules and compounds, their differences from elements, and the rules for representing and naming chemical substances. Understanding these concepts is fundamental to the study of chemistry, as compounds make up most of the substances encountered in everyday life.

The Properties of Compounds vs. Elements

• Compounds are substances composed of two or more elements chemically combined in fixed proportions.

• The properties of a compound are generally very different from the properties of the elements that compose it.

• Example: Table sugar (sucrose) contains carbon, hydrogen, and oxygen, but its properties differ greatly from those of graphite (carbon), hydrogen gas, or oxygen gas.

• Example: Sodium chloride (table salt) is made from sodium (a reactive metal) and chlorine (a poisonous yellow gas), yet salt is safe to eat.

Elements, Compounds, and Mixtures

• Most substances in nature are compounds, not elements.

• Free atoms are rare in nature.

• A compound is different from a mixture:

  • In a compound, elements combine in fixed, definite proportions.

  • In a mixture, elements combine in variable proportions.

Law of Constant Composition

• Formulated by Joseph Proust (1754–1826).

• Law of Constant Composition: All samples of a given compound have the same proportions of their constituent elements.

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

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

• This ratio is constant for all pure water samples.

• Example (Ammonia): Decomposing 17.0 g of ammonia yields 14.0 g of nitrogen and 3.0 g of hydrogen:

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

• Atoms combine in whole-number ratios, but mass ratios are not necessarily whole numbers.

Chemical Formulas: Representation of Compounds

• A chemical formula indicates the elements present in a compound and the relative number of atoms of each.

• Subscripts indicate the number of atoms; a subscript of 1 is omitted by convention.

• Examples:

  • Water: $H_2O$ (2 hydrogen, 1 oxygen)

  • Table salt: $NaCl$ (1 sodium, 1 chlorine)

  • Carbon dioxide: $CO_2$ (1 carbon, 2 oxygen)

  • Sucrose: $C_{12}H_{22}O_{11}$ (12 carbon, 22 hydrogen, 11 oxygen)

• Changing a subscript changes the compound entirely (e.g., $CO$ is carbon monoxide, $CO_2$ is carbon dioxide).

Order of Elements in Chemical Formulas

• Formulas list the most metallic element first.

• Among nonmetals, those to the left in the periodic table are listed before those to the right.

• Within a group, elements lower in the column are listed first.

• Table 5.1: Order of Listing Nonmetal Elements in a Chemical Formula

C	P	N	H	S	I	Br	Cl	O	F

• Exceptions exist, such as the hydroxide ion ($OH^-$).

Chemical Formulas: Polyatomic Ions

• Some compounds contain groups of atoms that act as a unit, called polyatomic ions.

• When more than one group is present, parentheses and subscripts are used (e.g., $Mg(NO_3)_2$).

• To find the total number of each atom, multiply the subscript outside the parentheses by the subscripts inside.

• Example: $Mg(NO_3)_2$ contains:

  • 1 Mg

  • 2 N (1 × 2)

  • 6 O (3 × 2)

Types of Chemical Formulas

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

• Molecular formula: Actual number of atoms (e.g., $H_2O_2$ for hydrogen peroxide).

• Structural formula: Shows how atoms are connected using lines for bonds.

• Molecular models: Ball-and-stick and space-filling models represent 3D structure.

Comparison of Formulas and Models (Example: Methane, $CH_4$)

• Molecular formula: $CH_4$ (1 carbon, 4 hydrogen)

• Structural formula: Shows each hydrogen bonded to carbon

• Ball-and-stick/space-filling models: Illustrate 3D geometry

Connecting the Macroscopic and Molecular Worlds

• Formulas and models connect what we see (macroscopic) with the atomic/molecular world.

• They are symbolic representations used by chemists to describe matter at the particle level.

A Molecular View of Elements and Compounds

• Pure substances can be elements or compounds.

• Elements may be atomic (single atoms) or molecular (diatomic or polyatomic molecules).

• Compounds may be molecular (covalent) or ionic (composed of ions).